Note: This game is also titled Dragon Ball Kai: Saiyajin Raishuu.
Find an area with strong battles. Defeat all but one enemy (or weaken the enemy if there is only one of them). Then, put everyone in the party except for the characters you want to level up on the sidelines. Have them finish the battle. All characters at the end of the battle will split the full experience point total equally, while characters on the sidelines always get at least 10% of the full experience, even if there are four or five characters outside the group at the end of the battle. This means there is actually a net increase in the experience points gained if you keep most of the characters out of the fight.
A good way to defeat him is to fight Wolfman before the final match. Get Goku up to level 10 before continuing so you inflict more damage with your melee attack. During the match with Piccolo Jr., you cannot use items. Do not use your Combination or Kamehameha techniques. Instead, use the After Image technique. This will increase your evading ability; thus, making you difficult to hit. Keep using your A. Guard and hitting him, and you should defeat him rather quickly.
Enter Baba's place, and climb the ladder to the right. Talk to the ghost. He will say "It's a secret to everybody" and give you 5,000 Zeni.