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.