The whole object of Super Mario 64 is to defeat Bowser. This page has my tips on beating him. There are 3 Bowser levels in the game. You have to beat him 3 times to win the game.
The first one is after you get 8 stars. Beat the level. Now go behind him, grab his tail (make sure you stopped running or else you'll dive right into him), swing him around using the control stick and press B again to let go. Try to throw him right into a bomb that is surrounding the platform. If you do this right, he should give you the key to the basement.
The second one is after you get 32 stars and beat Board Bowser's Sub in Dire, Dire Docks. Beat the level again and do the same in the first one. This is a little harder because he can put fire all over the place. Try to get coins from when the fire goes away and leaves them. If you beat this Bowser, he gives you the key to the upstairs.
The last Bowser is after you get 70 stars. Beat the level. Now you have to throw him into bombs 3 times. Don't go too far away from him or else he will run at you and it is hard to run away. If you beat this Bowser you can now watch the ending credits.
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