ok buddy i can tell your not the biggest mario fan so i will help you out... first when a plant comes out of the thinghy stand under it to catch it, then wait intill Wart (thats the name of the boss) is just about to blow bubbles at you and he opens his mouth for about 1 second and nothing comes out, chuck the plant at mouth (making him eat it) do this 7 times and you beat him!