If you are talking about Brawl, you use Bowser and get a Smash Ball, this is an item with the super smash bros. logo on it, to get it, you must break it by attacking it. When this is done, use the special attack button to release it and become Giga Bowser, or you can hack (ocarina which i have), In Melee, you can fight him if you beat adevnture mode on any difficulty except easy under 14 minuetes.