actually it's pretty easy. there's 2 secret prize boxes, one is that little fly and the other is that pink guy that flies. what works best for me is getting the fly first since he is smaller - that way you can jump on his head more. stay close to the ground cuz he follows you, when you start moving up you'll have more time to jump on his head... if you lose the fly then get the little pink guy to finish killing him. it's very easy

If you mean Plethora, you have to bounce on his head until his eyes start popping out of his head. Once all of his eyes are gone, he's dead. Sorta like the Shishkaboss, except Plethora takes a long time to kill.