If you're having trouble with punch out, here are some tips.
Don Flamenco: Don can be pretty easy to defeat. Attack when he taunts you, then dodge his uppercut. He will not always do this so stay on your guard. When he attacks, dodge and then go in for the uppercuts.
King Hippo: He can be really annoying sometimes because he blocks most of your attacks. When he opens his mouth, do an uppercut. His trunks will fall down. Go in for the kill and punch his stomach repeatedly. You only have to knock him down once and he will not get up. The reason mainly because he is obese. Watch for when he vibrates, because then he will open his mouth and try to punch you. You will have to react quickly though, for he attacks quickly.
Great Tiger: Watch the light on his headdress. When it flickers, he will jab at you. Dodge and strike back with an uppercut, but don't attack repeatedly, for you only get to hit him once here. The uppercuts are easy to dodge like always, just watch for when he crouches, dodge, and then uppercut him in the face. He has a special move that is really hard to stop. He will walk backwards and then circle around the ring, hitting you over and over again until you fall down. Block it four times in a row, but be careful because he moves very quickly. When you block it four times, he will be stunned and stand there with a dizzy look on his face. Land an uppercut to knock him down with one hit, no matter how much health he has left.
If anybody else knows how to defeat the other opponents, post the instructions in this thread.

I probably wont be much help I'm stuck on the Mexican boxer after I win the title. He is tough to hit.