Instructions
Strengthen your characters using stat points. Each time your level increases by one you earn three stat points that you can allocate as you choose. Open the menu with the "Start" button and press "A" on the status screen. Decide whether you want to increase your strength, defense or offense. Once you allocate the stat points you cannot change your mind.
Use a trick to increase the experience of your weaker characters. Control a stronger character and get an enemy to follow you to a save point. Kill the foe with this character and then switch out your players by using the "Switch Characters" option at the menu. The player you change to will get the experience for the kill.
Defeat Buu by using Super Saiyan 3 against him and then running away from him for 60 seconds on two separate occasions after his health wears down. You will have to press "A" rapidly to move a spirit bomb into Buu to ultimately destroy him.
Access the Kai Store after you beat Buu. After defeating the game's major foe you can visit Kai's planet. Go to the right part of the map and break through a wall where you can see a crack. This is the hidden Kai Store, which features strong armor and defensive gear that will help you level up your characters.