In Super Mario Bros.

How do I kill the dude at the end? I tried jumping on his head like the others but it's not working!!!

On which Super Mario Bros.?

Sounds like part 3... If so you have to get him to stomp through the blocks and fall down...

Yeah, you either get him to bust through the bricks til he falls in the pit, or you can get him with firePOWer.

Yup if u dont have fire POWer stand in the middle and wait till he jumps at you then move so he will fall through it.