From Bulbapedia, the community-driven Pokémon encyclopedia.
Mix Battle is a battle mode introduced in Generation IV and found at the Pokémon Communication Club Colosseum. Each player can only bring in three Pokémon. Before the battle, each player will choose one of the opponent's Pokémon, place it in their own party, and then battle the opponent.
Mix Battle mode can be found in all Generation IV games, including Pokémon Battle Revolution.
Pokémon Center Receptionist
- "Mix Battle is for two Trainers with three Pokémon each."
- "Each Trainer chooses one of the opponent's Pokémon, mixes them with their own party, and then battles the other Trainer."