Not very fun.
My first issue with this game is the perspective. I don't think a mouse controlled boxing game with this type of side scrolling only movement is a good thing. But, it doesn't work out horrible in this case.
The graphics aren't too great either. They could really be improved. I would work on the for your next flash. Maybe make an actual ring as well if you feel like it.
The sound effects aren't horrible, but you could add some type of music, because this game gets very tedious after a while.
Which brings me to my next problem, 5 rounds is more realistic, but it also makes this game very tedious. I think 3 rounds would be better.
I suggest that, because the zombie doesn't ever move. He merely stands there and waits for you to come to him, and then auto punches. He is very easy to beat by 50 points every single round.
The stamina bar you have never depletes, yet you can pay to have it replenished every single round. I was hit maybe 40 times in a round and it was still at 100. I would work on fixing that.