Haha Dan, beating King Hippo isn't that tough if you have the know-how. Wait until he opens his big fat mouth to do a super punch on you, and hit him square in the jaw. It might take a few tries, but if you do it, it'll give you a chance to punch his belly. You can get like 7 or 8 body shots in before he blocks you. Repeat that four times and he'll be down for the count. He's the only guy you're guareneteed a KO on.
As for the second Piston Honda, if you punch him in the gut right before he unleashes his piston punches, it'll be an automatic KO. It's tricky, but can be done. I always have a problem with Mike Tyson. Even if you survive his first onslaught of automatic knockdown upper cuts, best of damn luck getting a TKO.