Co-Op Event 2: Master the Pokémon Tag Battle
From SmashWiki, the Super Smash Bros. wiki
Master the Pokémon Tag Battle (きわめろ！ポケモンタッグバトル, Master! Pokémon Tag Battle) is the 2nd co-op event match out of a total of 21 in Super Smash Bros. Brawl. It involves player 1 controlling the Pokémon Trainer with 3 stock (like in Event 5: Become the Champion!) and player 2 controlling Pikachu with 1 stock to defeat another Pikachu and Pokémon Trainer. The fight takes place on Pokémon Stadium 2. Interestingly, when one of the Pokémon Trainer's Pokémon is KO'd, they can not be used again like it normally can.