To easily gain experience, find an area with strong battles and defeat all enemies except for one. Then, put everyone in the party except the characters you want to level up on the sidelines. Kill the remaining enemy. All characters in battle will split the experience point total, while all characters on the sidelines will receive 10% of their full experience regardless of the number of characters on the sidelines. So you actually gain more experience by keeping most of the fighters on the sidelines.
When using the following capsules on standby, theyy are only 50% effective: Chi-Chi's Lunch, Korin's Staff, Heavy Karate Outfit and/or Oolong's Wallet. The Spark Boots halves their effectiveness again to 25% because it doubles the length of your steps. Don't use the Spark Boots when trying to use these items to their fullest effect.
You can't use items during the match. First, fight Wolfman before the final match until you get Goku up to level 10. You'll do more damage with your melee attack. When fighting with Piccolo Jr., don't use your Combination or Kamehameha techniques. Instead, use the After Image technique to increase your evading ability. You'll be more difficult to hit. Guard and pounding on him and you should defeat him in no time.