Mewtwo, The Genetic Pokémon. Mewtwo is a Pokémon that was created by genetic manipulation. However, even though the scientific power of humans created this Pokémon's body, they failed to endow Mewtwo with a compassionate heart. A Pokémon created by recombining Mew's genes. It's said to have the most savage heart among Pokémon.
For the longest time, Mewtwo stood as the single most powerful Pokémon, representing both an a achievement and living nightmare of scientific progress. Nothing could stand up to Mewtwo and hope to win alone, and the combination of Psychic's few resistances and Recover made defeating Mewtwo a losing battle. The goal of creating the world's most powerful was seemingly achieved. However, as more Pokémon began to be discovered it became more apparent that Mewtwo was not quite all-powerful, and that the gods and super titans of the Pokémon world could easily stand up to or even outmatch this monstrosity of scientific progress. Meanwhile its title of “world's most powerful Pokémon” was found to already belong to a sky dragon that preceded Mewtwo's existence by several hundred million years. While time has exposed that Mewtwo is not quite the unstoppable monster that people once believed it to be, it's still an incredibly powerful threat that should never be taken lightly. Incredible Speed and Special Attack are its two most distinguishing traits and not many Pokémon can match up to that power, while a huge movepool lets it threaten a wide variety of threats. Mewtwo also stands as one of two overwhelmingly powerful legendaries to gain the power of Mega Evolution, and one of two Pokémon in general to obtain two different Mega Evolutions. All of this undoubtedly makes for a huge threat, but there are several factors that continue to keep Mewtwo in check. Most significantly, Mewtwo's Psychic typing is no longer the same dangerous type it was back in during the years of its creation. Weaknesses to Dark and Ghost are very problematic since moves like Sucker Punch and Shadow Sneak overcome Mewtwo's fantastic speed while picking away at its less than spectacular bulk, and Pursuit makes it regret trying to escape from a dangerous situation. Furthermore it lacks many resistances to dangerous types which makes it difficult for Mewtwo to directly switch into powerful threats in a world where everything hits equally as hard as Mewtwo itself. Make no mistake though, Mewtwo still certainly has a place in a world dominated by gods and titans like Arceus, Mega Rayquaza, Xerneas, Primal Groudon and Primal Kyogre, though the days of Mewtwo laying waste to everything in its path are long gone.
+ Incredible base 154 Special Attack is among the highest in the game, only outdone by Mega Evolutions and other legendaries. A fantastic special movepool including Ice Beam, Focus Blast, Fire Blast and its signature move Psystrike complements this power, letting it threaten many Pokémon.
+ With base 130 Speed, Mewtwo is one of the fastest Pokémon in the game ignoring Choice Scarf users. Given Mewtwo's less than impressive bulk, this is important to Mewtwo's success as a special attacker.
+ Mewtwo has no shortage of useful support and setup moves like Taunt, Calm Mind and Will-O-Wisp to help diversify its sets, making it difficult to predict what Mewtwo is running.
+ Mewtwo has two different Mega Evolutions, which contributes to Mewtwo's incredible diversity in moveset options. Unlike Rayquaza, neither of its Megas proved to be too good to end up banned under certain rulesets.
- Psychic typing lacks many useful resistances making it difficult to switch in, made more problematic by Mewtwo's average 106/90/90 bulk. No major threat is weak to Psychic either so it often relies on coverage to hit threats, which isn't always enough.
- Even though Mewtwo has coverage moves for most threats, anything that it can't OHKO threatens Mewtwo greatly, which is much more realistic now that there are many threats bulky enough to take a hit or two from it.
- While Mewtwo has two Mega Evolutions, neither of them are significantly better than its base form, so it often competes with better Megas like Rayquaza and Salamence for the Mega slot.
- Across its regular form and both of its Megas, Mewtwo has no good abilities. All of its abilities are niche at best, although generally its abilities won't come into play.
- The first generation of Pokémon is long gone. In other words, there are far more threats to Mewtwo than just Chansey these days.
Pressure: When this Pokémon is hit by a move, the opponent's PP lowers by 2 rather than 1. This isn't a bad ability, but it's not a particularly good one either. Pressure has the possibility of draining low PP moves quickly which are not uncommon among legendaries, but if Mewtwo is getting hit it usually isn't lasting long enough for its team to rely on this.
Hidden Ability (Available through transfer):
Unnerve: The opponent Pokémon will never use their held Berry while the Pokémon is in battle. In theory this could be useful to deal with Pokémon that might be running berries to avoid a super-effective hit, but this isn't happening often enough for it to be very useful. It is also not easy to obtain so you're usually better off with Pressure since that will at least have an effect on everything.
- Focus Blast / Aura Sphere
- Ice Beam / Shadow Ball
- Fire Blast / Taunt
Item Attached: Life Orb
EVs and Nature:
EVs: 4 Def / 252 SpA / 252 Spe
This set takes advantage of Mewtwo's impressive special movepool to function both as a special wallbreaker and late-game cleaner. Psystrike is Mewtwo's signature move and strongest STAB move, functioning as a stronger equivalent of Psyshock. Focus Blast provides necessary Fighting-type coverage to hit Dark-types like Tyranitar, Dark Arceus and Darkrai while hitting Steel-types like Dialga. Aura Sphere is an alternative if you don't want to risk the low accuracy, but the power difference is massive and it fails to reliably beat most of the aforementioned threats anyway so you're usually better off with Focus Blast. Ice Beam lets Mewtwo threaten the multitude of Flying-types like Rayquaza, Salamence, Lugia and Yveltal while also hitting Ground Arceus, Latios and Latias. Shadow Ball can be used instead to hit opposing Mewtwo, Ghost Arceus and the occasional Aegislah while still being able to ht Lugia, Latios and Latias. However, the ability to hit Mega Salamence is typically too useful to pass up. Fire Blast is used to hit Klefki who only takes neutral damage from Focus Blast, though it also hits Ferrothorn, Scizor and Aegislash harder than any of Mewtwo's other moves do. Taunt gives up coverage which might sound contrary to what Mewtwo wants, but it's useful for preventing status moves and helps it against bulkier teams.
Mewtwo wants to hit as fast and hard as possible, so it should always be running with maximum Special Attack and Speed investment. The last 4 EVs may seem trivial but due to its equal defenses, the remaining EVs are placed into Defense to prevent Genesect from getting an Attack boost from Download. Timid nature takes advantage of its impressive base 130 Speed, and it's necessary in order to outspeed Mega Salamence, Darkrai and faster Arceus formes. Life Orb is also recommended to further Mewtwo's damage output, and since Mewtwo isn't meant to be taking hits the recoil is not too problematic.
- Calm Mind is an option to give Mewtwo an opportunity to sweep more easily since it often finds itself missing out on OHKOs against many threats. This is an effective alternative since Mewtwo naturally outspeeds most of Pokémon naturally, though it's very reliant on forced switches to set up since Mewtwo is not bulky enough to set up against many threats.
- Electro Ball is a very uncommon option that has very little use, but with a Calm Mind boost it can OHKO Mega Sableye. This is a very niche option though and has no other relevant use.
- Substitute can be used to punish switches and protect Mewtwo from status from the likes of Klefki and bulky Arceus formes. However, running it alongside Life Orb causes Mewtwo to be worn down very fast.
- Mewtwo can opt to run a Choice Scarf set despite its already high speed, in order to function as a check to setup sweepers like Rayquaza and Mega Salamence even after one Dragon Dance. However, doing so makes it difficult for Mewtwo to abuse its great coverage.
- A bulkier set with Psystrike, Will-O-Wisp, Taunt and Recover while holding Leftovers is viable, but threats like Primal Groudon, Mega Sableye and Yveltal makes this difficult to use.
Unlike in the old days, using your own Mewtwo and Chansey is no longer the only way to deal with Mewtwo. With more powerful Ghost-types and Dark-types there are far more answers to Mewtwo, though with its wide coverage options it's difficult to find something that completely counters it. Origin Forme Giratina and Aegislash can threaten to revenge kill Mewtwo with Shadow Sneak, and both are bulky enough to come in relatively safely on most of Mewtwo's moves other than Shadow Ball or Fire Blast, respectively. Similarly, Ghost Arceus threatens Mewtwo with Judgment though it takes heavy damage from Psystrike and Shadow Ball so it must be careful switching into it. Mega Sableye can also handle most Mewtwo variants can bounces Taunt back at it, though Calm Mind Mewtwo can be problematic. Yveltal, Tyranitar and Dark Arceus can also threaten Mewtwo with their respective Dark-type moves, with Yveltal notably carrying Sucker Punch to revenge kill it. Chople Berry Tyranitar can also Pursuit trap Mewtwo, which makes it difficult for Mewtwo to get out of the situation. However, all of these Dark-types are threatened by either Ice Beam or Focus Blast so they must be careful taking a big hit upon switching in. Just as Mewtwo had a difficult time fighting itself in the first generation, Mewtwo still doesn't fare too well against opposing Psychic-types so Psychic Arceus and Lugia can come in relatively safely, only fearing the occasional Shadow Ball Mewtwo with Calm Mind. In return both can cripple it with status or begin to set up themselves, though Taunt Mewtwo can throw off those plans. Beyond type advantage, Mewtwo is very suspective to revenge killers since it has no way to avoid priority moves amidst all the Life Orb recoil it takes on top of its average bulk. Choice Scarf Genesect, Sucker Punch users like Yveltal and Kangaskhan, and Extreme Speed users like Rayquaza and Arceus can all come in and pick off Mewtwo once it's been sufficiently worn down. Mewtwo is also not a fan of status moves since paralysis neutralizes one of Mewtwo's most threatening traits, while poison and even burn damage will quickly wear it down. Taking all of these checks aside, Mewtwo in general still lacks the power to OHKO many of the bulky threats that have emerged without prior damage, so if Mewtwo's foe survives then Mewtwo faces heavy retaliation. Mewtwo is still a terrifying threat that can easily tear through the opposing team if you aren't careful, but there is no shortage of options to keep Mewtwo in check.
As one of two Pokémon to obtain two different Mega Evolutions, Mewtwo manages to find ways to further diversify its abilities. As with any Mega Evolution, the power boost raises Mewtwo's base stat total to a staggering 780, surpassing all other Pokémon and only being matched by a divine being known as Mega Rayquaza. Mega Mewtwo X in particular completely changes Mewtwo's fighting style, giving it a greater focus on physical power and turning it into a dual typed Psychic/Fighting Pokémon. Unlike most Psychic-types, Mewtwo also has the physical movepool to support a physically based Mega Evolution, so the unpredictability of whether it runs a physical set or special set makes it a bigger threat than it already was. Moves like Low Kick, Zen Headbutt, the elemental punches and even rarities like Self-Destruct give Mewtwo plenty of options to threaten opponents just as it would with special moves, and with one of the highest Attack stats in the game it has more than enough power to take full advantage of its newfound power. Mega Mewtwo X also has the added bonus of maintaining its naturally high Special Attack, so it's still capable of throwing its opponents off-guard with special moves like Focus Blast, adding an extra layer to the mindgames that Mewtwo already provides. However, despite the incredible power that its Mega Evolution provides, Mega Mewtwo X is not without notable shortcomings. While a Psychic/Fighting typing grants it a very useful neutrality to Bug and Dark type moves, added weaknesses to Flying and Fairy are very dangerous in a world dominated by Xerneas, Mega Salamence, and on an even higher plane, Mega Rayquaza. In addition, it faces competition for the Mega slot by more threatening Megas such as the aforementioned Mega Salamence and Mega Rayquaza which boast a more useful Dragon/Flying typing. It still lacks many resistances and is not much bulkier than normal, so powerful neutral attacks still put Mewtwo at risk of taking huge damage. Its STAB moves are also varying in usefulness since Low Kick doesn't do much damage to lighter opponents and Zen Headbutt is not a particularly strong move, so while they're effective at hitting a select group of Pokémon this also makes it easier to wall with options like Lugia, Giratina, Aegislash and Mega Sableye around. Shortcomings aside though, Mega Mewtwo X adds a dangerous layer of diversity to Mewtwo's impressive repertoire, and it's one you should never ignore when facing this genetic monstrosity.
+ An added Fighting typing grants Mewtwo a useful secondary STAB move to handle a wider variety of threats more effectively. It also makes it less susceptible to Dark and Bug type moves at the cost of now being weak to Flying and Fairy types.
+ Mega Mewtwo X boasts an incredible base 190 Attack, placing it as the Pokémon with the highest base Attack stat in the game. Moves like Low Kick, Zen Headbutt and the elemental punches complement this huge Attack very well.
+ Maintaining its natural base 154 Special Attack means it can still opt to run special moves and abuse its better offensive typing with moves like Focus Blast.
+ Mewtwo is still fast, base 130 Speed means that it still naturally outspeeds most of the relevant threats to it.
- Weaknesses to Flying and Fairy types means it's now weak to Xerneas, Mega Salamence and Mega Rayquaza, which are all much more relevant and bigger threats than any Dark-type or Bug-type it would have been weak to otherwise.
- Low Kick situationally useful, which means that its STAB moves are far weaker against Pokémon that aren't heavy enough to take huge damage from Low Kick. Thankfully most legendaries are naturally heavy, though against threats like Klefki this is problematic.
- Most of its physical moves have lower base power than its special moves, so even with its massive Attack stat it finds itself missing out on numerous KOs.
- The competition for the Mega slot is fierce, especially with Mega Salamence and Mega Rayquaza around, so it tends to be overlooked in favor of these Megas. The fact that these two are much harder to check than Mega Mewtwo X does it no favors.
Steadfast: Speed raises by one level every time the Pokémon flinches. This is generally not a very useful ability, as Mewtwo is already naturally faster than most Pokémon and flinching is completely dependent on secondary effects that won't always activate. If faced with Choice Scarf users with moves that flinch then Steadfast can come into effect, though these situations are not common.
- Low Kick
- Taunt / Zen Headbutt
- Ice Punch / Stone Edge
Item Attached: Mewtwonite X
Ability: Steadfast (Pressure on Mewtwo)
EVs and Nature:
EVs: 4 HP / 252 Att / 252 Spe
Mega Mewtwo X doesn't have a better Fighting-type move than Low Kick, but against most legendaries that don't resist it, Low Kick is Mewtwo's best physical STAB move. This is notably useful for beating Normal Arceus, which is one of the draws of using Mega Mewtwo X at all. However, since the power is dependent on weight it falls short against light threats like Klefki, so Earthquake is used to cover some of these threats. Earthquake also notably hits Mega Gengar, Aegislash, Diancie and Primal Groudon, providing very useful coverage. Taunt is typically used over another attacking move because as with Life Orb Mewtwo, this set wants to avoid status especially now that it's more vulnerable to burns. This set is also troubled more by physical walls so being able to at least Taunt them is useful. However, Zen Headbutt can be used as a secondary STAB move for neutral coverage against Ghost-types while being Mewtwo's best physical Psychic move. That being said there are no relevant threats that are hit by Zen Headbutt so Taunt is typically the better option. The final move is used to give Mewtwo a way to deal with the Flying-types that often wall it otherwise, with Ice Punch being the preferred option to hit Rayquaza, Latios, Latias and Mega Salamence. Stone Edge is an alternative that still hits Rayquaza and Salamence while also hitting Ho-Oh, though the choice ultimately depends on which threats are more important to hit for Mewtwo's team.
Mewtwo needs Mewtwonite X in order to Mega Evolve into its physically inclined form. Jolly nature is preferred to ensure Mewtwo still outspeeds all relevant threats such as Darkrai and Arceus, and Mewtwo's Attack stat is high enough to where Adamant isn't really necessary. Maximum Speed investment ensures that Mewtwo is as fast as possible and maximum Attack investment makes full use of its incredible Attack. However, an alternative spread of 112 Att / 144 Def / 252 Spe gives up some of its power to ensure that +2 Extreme Speed Arceus can't OHKO Mega Mewtwo X even after Stealth Rock damage, which allows it to effectively check this Arceus variant.
- Ice Beam can be used over Ice Punch to hit Salamence harder and bypass the Intimidate drop. If this is used then a Naive nature is preferred to let Mewtwo make effective use of its powerful base 154 Special Attack.
- Mega Mewtwo X can make use of Substitute to shield itself from status and punish switches. However, most walls give this Mewtwo variant a tough time anyway so it still has a tough time beating most of them even with a Substitute.
- Iron Tail lets Mewtwo hit Fairy-types like Xerneas and Diancie hard since it has trouble beating them and risks Xerneas OHKOing it otherwise. Poison Jab is a weaker but more accurate alternative that still hits Xerneas, but it doesn't hit Diancie.
- Mewtwo can utilize a Bulk Up set with Drain Punch to function as a sturdy win condition, though Drain Punch is weak without boosts and it can be difficult to find setup opportunities.
- Mega Mewtwo X can opt to run a special set to abuse its better typing for STAB Focus Blast, though this is usually not the most effective use of the Mega slot.
Countering Mega Mewtwo X
Mega Mewtwo X is tricky to counter because you won't know you're up against it until after it has Mega Evolved. However, its checks don't change too much between any of its forms so you can typically use the same things to check Mega Mewtwo X. Bulky Psychic-types remain problematic, as Lugia and Psychic Arceus generally don't mind much of what Mewtwo can do. Lugia needs to be careful with Stone Edge or Ice Punch if Multiscale is not intact, though otherwise it can threaten it with Aeroblast. Flying-types in general give this variant of Mewtwo some trouble, as Ho-Oh doesn't mind dealing with Mewtwo lacking Stone Edge while Rayquaza and Mega Salamence threaten it with Dragon Ascent and Aerilate Double-Edge respectively. Rayquaza and Mega Salamence can only check it if they get a chance to setup beforehand though, as they're otherwise outsped and KOed by Ice Punch with the exception of Mega Rayquaza under Strong Winds. Yveltal might not like Low Kick, Stone Edge or Ice Punch, but despite the Fighting typing it still threatens Mega Mewtwo X with Foul Play and Oblivion Wing. Ghost-types that threaten Life Orb Mewtwo still threaten Mega Mewtwo X all the same, as Ghost Arceus, Giratina, Aegislash and Mega Sableye are immune to Low Kick and still don't mind Zen Headbutt if Mewtwo runs it. However, Giratina and Aegislash still take super-effective damage from Ice Punch and Earthquake respectively so they still need to be careful when switching into Mewtwo. Fairy-types lke Xerneas, Klefki, Clefable and Diancie also give Mewtwo issues since Mewtwo struggles to beat them unless it runs Iron Tail or Poison Jab. Xerneas in particular threatens to use Mewtwo's switch as an opportunity to set up with Geomancy, and Clefable stops Bulk Up variants completely. Meanwhile Klefki takes little damage from Low Kick and can threaten to hit it with Prankster Thunder Wave, though it must be careful of Earthquke. This variant of Mewtwo is also more susceptible to status now that burns effectively neuter its damage output, so status users with Thunder Wave, Will-O-Wisp and Toxic are very problematic for Mewtwo. Aside from Klefki, effective options include support Arceus formes, Mega Sableye and Giratina, which are all options that check Mega Mewtwo X effectively by virtue of typings. Many of these checks also deal with other variants of Mewtwo, so covering Mega Mewtwo X isn't overbearing despite its newfound Psychic/Fighting typing.
Originally revealed as Mewtwo's awakened form, Mega Mewtwo Y can easily be described as such. This Mega Evolution takes Mewtwo's famed Special Attack and Speed and boosts them even further, making for a lightning fast and incredibly powerful special attacker, claiming the title of highest Special Attack among all Pokémon. Everything that was true about Mewtwo before still stands, its wide movepool helps it threaten a huge range of threats and its Speed is almost unmatched, with a small boost to take it even further. A boost to its Special Defense also makes it a more effective Calm Mind user than it would be in its normal form. However, as with many Megas introduced before ORAS, Mega Mewtwo Y obtained a few questionable stat changes that serve to hurt it more than anything. A significant boost in Attack does no favors for Mewtwo, only making it more vulnerable to Foul Play, while a decreased Defense stat means it has even more trouble taking physical hits than it already does, especially with powerful priority users to bypass Mewtwo's incredible Speed. Furthermore, all of the problems that trouble Life Orb Mewtwo still hold true, so its typing means it has trouble finding switch-in opportunities while weaknesses to Shadow Sneak and Sucker Punch means it still needs to be careful around Ghost and Dark types. As with its Fighting-type counterpart, Mega Mewtwo Y also faces competition for the Mega slot, which becomes even more difficult when Life Orb Mewtwo hits slightly harder than its Mega Evolution does without using the Mega slot. However, that isn't to say that Mega Mewtwo Y is worse than its normal form, as the speed, power and special bulk go a long way in giving Mewtwo justification to use its awakened form. Even with the competition and weaknesses it has, Mega Mewtwo Y is still a worthy advancement from this genetic experiment's creation. It might not descend from the heavens to lay waste to everything its path the same way Mega Rayquaza does now, but Mega Mewtwo Y is not a threat you should ever take lightly.
+ With a base 194 Special Attack, Mega Mewtwo Y has the highest Special Attack among all Pokémon. Needless to say, Mega Mewtwo Y hits very hard.
+ Mewtwo's frighteningly large movepool makes for an incredibly dangerous special attacker, giving it coverage moves to hit just about anything.
+ Base 120 Special Defense makes Mega Mewtwo Y a surprisingly sturdy Calm Mind user. This combined with its base 140 Speed makes for a very dangerous sweeper that can be very difficult to take down if you lack physical priority moves.
- Base 70 Defense is frail by any standards, and with a Psychic typing this means that physical priority moves are a huge problem. This doesn't help the fact that it lacks many useful resistances in the first place.
- For some unknown reason it gets +40 Attack upon Mega Evolution, so a good chunk of its boost is essentially useless.
- It faces huge competiton from Mega Salamence and Mega Rayquaza for the Mega slot, while also competiting with its normal form which hits harder when equipped with a Life Orb.
- Depending on who you are, you might not be a fan of its new voice it got in the anime to go along with this Mega Evolution.
Insomnia: The Pokémon cannot be put to Sleep while having this ability. This ability has its uses, particularly in preventing Darkrai from using Dark Void against Mewtwo. However, Darkrai is the only relevant user of sleep moves in Singles and can threaten to hit Mewtwo with Dark Pulse anyway so don't rely too much on Mega Mewtwo Y to be your Darkrai check.
- Focus Blast / Aura Sphere
- Ice Beam / Shadow Ball / Calm Mind
- Fire Blast / Taunt / Calm Mind
Item Attached: Mewtwonite Y
Ability: Insomnia (Pressure on Mewtwo)
EVs and Nature:
EVs: 4 Def / 252 SpA / 252 Spe
The first thing that might stand out is that this set looks very similar to the Life Orb Mewtwo set, and that's because it is. Mega Mewtwo Y essentially works the same way that Life Orb Mewtwo does, just with slightly more Speed and Special Defense. Psystrike is the mandatory STAB move and it's Mewtwo's strongest STAB move. Focus Blast provides coverage to hit Steel-types like Ferrothorn and Dialga, but most importantly it lets Mewtwo beat Tyranitar, Dark Arceus and Darkrai. Aura Sphere is an alternative that never misses if you're not a fan of the Focus Blast's shaky accuracy, though the huge power drop typically makes it an inferior choice. Ice Beam lets Mewtwo cover Dragon types like Rayquaza, Mega Salamence, Latios and Latias, so it tends to be the most useful coverage option in the third slot. Shadow Ball can be used to hit Ghost-types instead such as Aegislash, Giratina and Ghost Arceus, though usually Mewtwo is better off using another move against them. Mega Mewtwo can run Calm Mind to make for an effective setup sweeper thanks to its impressive special bulk, and it can be difficult to stop without priority thanks to its incredible speed. Fire Blast is used to hit Steel types that don't mind Focus Blast, such as Klefki, Aegislash and Scizor, while still hitting Ferrothorn. However, Taunt can be used in conjunction with Calm Mind to threaten defensive teams more effectively.
Mewtwo naturally needs Mewtwonite Y to make use of this Mega Evolution, though the EV spread and nature function the same as Life Orb Mewtwo. A Timid nature is preferred to take full advantage of its incredible Speed tier, and its Special Attack is so high that it doesn't need to power boost too much especially if it runs Calm Mind. Maximum Special Attack and Speed investment take full advantage of its most threatening stats, allowing it to hit as hard and fast as possible. Insomnia is the only ability Mega Mewtwo Y can use, though making Darkrai think twice about using Dark Void against its team can be useful.
- Recover can be used alongside Calm Mind to increase Mewtwo's longevity, though doing so gives up coverage while still leaving it susceptible to status.
- Electro Ball alongside Calm Minda can be used to threaten to OHKO Mega Sableye, though this has no reasonable use otherwise and is difficult to obtain anyway.
- Substitute can be used to give Mewtwo a buffer against status, and it punishes switches very well due to its high Speed which makes breaking the Substitute a dangerous task. It also serves as a buffer against a possible revenge killer which is useful due to its poor Defense.
Countering Mega Mewtwo Y
While the power of Mega Evolution may have given Mewtwo plenty of new options, even Mega Evolution couldn't bring it back to its former glory. Dealing with Mega Mewtwo Y is virtually the same as dealing with Life Orb Mewtwo. Everything that worked against Mewtwo before still works against it, although now it's even more vulnerable to being revenge killed by priority moves. Normal Arceus and Rayquaza can offer this priority in the form of Extreme Speed, while Mega Kangaskhan, Yveltal, Origin Forme Giratina and Aegislash have Sucker Punch and Shadow Sneak respectively. Beyond simply priority, any physical attacker that can take a hit from Mewtwo and retaliate back gives it trouble, since the low Defense makes it difficult to handle most powerful physical attacks. However, unlike with Life Orb Mewtwo, special attackers can't rely on this due to its respectable base 120 Special Defense giving it reasonable bulk on that side of the damage spectrum. Dark-types still give Mega Mewtwo Y trouble so Yveltal, Dark Arceus and Tyranitar are generally good checks. None of these enjoy switching into Focus Blast, or Ice Beam in Yveltal's case, so they must be careful when switching in. Meanwhile Ghost-types still deal with Mewtwo relatively well, as they only really dislike Shadow Ball or switching into Psystrike. Giratina, Aegislash, Mega Sableye and Ghost Arceus are all good options for this, though Mega Sableye must be extra careful with Calm Mind since Mega Mewtwo Y has extra bulk that might make it tougher to win a Calm Mind war with it. Bulky Psychic-types also remain effective as answers to Mega Mewtwo Y, making Lugia and Psychic Arceus good options for beating it. Lugia in particular can Whirlwind Mewtwo out if it tries to set up with Calm Mind, though it has to be careful taking boosted Ice Beams if Multiscale is not intact. As always, Mewtwo is also does not deal with with status since it's incredibly reliant on its Speed and being worn down fast can quickly put it in range of being picked off by priority moves. Klefki is a good status user that gives Mewtwo trouble if it lacks Fire Blast, and even if it does carry Fire Blast it can usually fire off a Prankster Thunder Wave before it goes down. Mega Sableye and most support Arceus formes are also good status users that can stop Mewtwo as well. Many of these answers typically have to be careful of one coverage move or another which can make Mega Mewtwo Y difficult to deal with, but it can never cover everything so once you figure out what it doesn't have, bringing in the appropriate check is not a big problem.
Locations in Games
Trade from FireRed/LeafGreen
Trade from FireRed/LeafGreen
Trade from HeartGold/SoulSilver
Transfer from Generation IV
Black 2/White 2:
Transfer from Generation IV
Omega Ruby/Alpha Sapphire:
Trade from X/Y