After you have defeated the Grand Masters for the first time, when you go to Dr. Mason's laboratory, you will find that he has activated a special machine known as the Challenge Machine. In the Challenge Machine, you will face off against 5 random trainers with no break and no opportunity to change your deck. However, you luckily get to see the five trainers before you begin.
The first trainers go as follows. Rounds 1 through 3 are just standard Club Trainers that you will have encountered. Round 4 is usually a Club Master, while the final round is a Grand Master, or occasionally Imakuni or Aaron.