Instructions
Fight Kid Buu as Goku. The game will automatically choose Goku as your first fighter against Kid Buu. He will already be at Super Saiyan 3. Do not pin Kid Buu to the wall and attack him relentlessly as you would do in other fights; he will teleport behind you and hit you with a Kamehameha wave. Instead, engage him with a series of quick attacks in the middle of the fight area until you've taken away a fifth of his health.
Fight Kid Buu as Vegeta. When you've reduced Kid Buu's health by a fifth, Vegeta will take Goku's place. Continue attacking Buu with light, quick attacks, but be warned: Vegeta does not get to become a Super Saiyan in this fight, making him considerably weaker and slower than Goku. The fight will continue until you've reduced Kid Buu's health by half.
Fight Kid Buu as Goku again. Goku will step back in, but this time at a normal power level rather than as a Super Saiyan 3. The goal this time isn't to damage Kid Buu as it is to fill Goku's Super Saiyan meter. Attack Kid Buu furiously to fill the bar, while doing your best to avoid his attacks.
Power up to Super Saiyan 3 when Goku's power meter is full. Kid Buu will now split into three and begin attacking you relentlessly with all three bodies. Block often, attack whenever you can. All three Kid Buu's take damage, so attack whenever you can isolate one. Once again, you only have to knock off a fifth of his health until you switch fighters again.
Fight Kid Buu as Vegeta when he steps in again. You cannot win this portion of the fight as Vegeta. Instead, you have to simply survive for 60 seconds. Run often, block more, and only attack in order to escape when one of the three Kid Buu's corners you.
Survive another 60 seconds fighting Kid Buu as Vegeta after watching the extensive cutscene. The same strategy as before works.
Play the mini-game and win. After Vegeta survives the second 60 seconds with Kid Buu, Goku will launch a massive Spirit Bomb at him. Press the "A" button as fast as you can. Once the bomb connects, Kid Buu will die and you've beaten the game.