AH, you're challenging Radomus. Radomus is a Doubles leader whose strategy focuses around Trick Room. His team is bulky and hits hard, and if he sets up, then it's going to be rough going. But if you can stop him from using Trick Room, then he might as well be a sitting duck.
An another note, neither side is alone in this battle. The chess pieces may come into play too. Radomus can move them with his psychic attacks, dealing extra damage, but you might be able to find ways to move them too. Keep in mind that like any good chess game, Pokemon with high intellect or strategy-based attacks will do well here.
Good luck!
Trainers
Cain is battled on a Chess Board in a Single Battle format and he has two Moomoo Milks.