Managing your Pokémon party effectively is crucial for success in Pokémon Colosseum, especially given the emphasis on Double Battles. You can have up to six Pokémon in your party at any time, but only two can be active in battle. Aim for a balanced team that covers a wide range of types and has a mix of physical and special attackers, as well as defensive Pokémon.
Consider the roles each Pokémon will play. For example, you might want a fast attacker, a bulky defender, a Pokémon with status-inflicting moves, and a Pokémon with strong offensive coverage. When facing a specific opponent, you may need to swap Pokémon in and out to exploit type advantages or counter their strategies. Don't hesitate to swap out a Pokémon that is at a disadvantage to bring in a more suitable matchup. Regularly check your Pokémon's movesets and consider teaching them new, more powerful moves as they level up or through Technical Machines (TMs).