Instructions
Attack Kid Buu one shot at a time. Press "A" to perform a melee attack or "B" for an energy attack. Avoid hitting him more than once at a time, or else Buu will hit you and take off a considerable amount of health.
Back away after every shot with the directional pad. Buu will attack but miss as long as you are out of the way.
Repeat steps 1 and 2 until Buu only has a fifth of his health left. You can see his health bar decrease on your screen. After he loses this amount of health, a small cut scene will play, and a timer will appear on-screen.
Avoid Buu's attacks for the next 60 seconds. You need not worry about hitting him; surviving the 60 seconds is the key to defeating him.