First when you jump down onto the screen you will straight away notice a overgrown blueberry hanging from a vine behind the boss. Your aim is to hit that vine so it swings and hits the boss. ( 10 times) Easier sed than done.
First you hav to stun the boss wen he walks up to u duck let go then jump NOW is your chance to punch him this will make him dizzy and walk round in circles jump over him and then charge your punch then hit the vine it should smack him one across his face with a really big sound effect. But that aint enuff do this 9 more times and he will be knocked out.

I hope this helped you a lot it worked for me so it should for you.

oh by the way: wen the credits appear hold the A button for atleast 3 minutes TRUST ME this DOES work so don't worry. It is worth it you get a extra 5 secret levels and you can choose the last bosses you battled or the main boss they all come with their own special gadgets or powers. ENJOOOOOOY