Fiery Field

Final Floor: 30F
Unlock Method: Speak to Spinda after going to Solar Cave and get the Clear Wing, then visit Xatu in Grand Canyon
Initial Level: -
Maximum Teammates: 3
Items: Yes
Check Map: No
Opponents Able to Join: Yes

You'll first encounter Entei in Fiery Field after you encounter Spinda in the Pokémon Square. It will tell you of a Mirage Pokémon that it has seen and gives you a Clear Wing. Your first encounter has it enfuse colour into the Wing, but after you have recruited Ho-Oh, you can return there and battle it again. When you do, it will join you and, if you haven't already received it, give you the Sacred Field Camp