First, use Jigglypuff. Fly over Giga bowser and just when you touch him use your rest attack (B+down) and he will go flying. It usually takes about 2 times to knock him out. Repeat this until he dies. Then Gannondorf and Mewtwo will be there with 3 lives. Run through the stage back and forth looking for items to throw at them and soon you will win.