Instructions
Select Adventure Mode from the main menu. To make it as easy as possible, choose the difficulty "Very Easy" and a stock of "5." On a low difficulty, you can beat your enemies with fewer punches. With a stock of five, you die five times without having to continue. If you continue, you can't get Giga Bowser.
Proceed through the first 21 stages of Adventure Mode. Giga Bowser is the final boss. You fight him on level 22.
Double-jump to reach Giga Bowser's head. You can hit him there for good damage.
Grab items before Giga Bowser reaches them. In the middle of the fight, he'll often pause and start moving toward an attack item. If he gets it, you're in trouble. Get it before he does, and then throw it off the stage.
Continue attacking Giga Bowser, until he goes down. He packs a powerful punch, but he's slow. The trick to beating him is to outrun his attacks. After evading an assault, immediately counterattack. Once you've hit him dozens of times, he'll fall like all the opponents before him.
Select Giga Bowser from the character selection screen. You can play as him during multiplayer bouts, though with his immense strength and stamina, he's somewhat unbalanced.