I managed to clear the game by picking the shirtless character with the red beads around his neck and spamming the kick move into the edge of the screen. The opponent A.I would keep walking into me and they'd be stuck in a damage loop as I'd keep getting hits in.
You need to program it so that the player cannot continue to attack after the first round it over. I was able to keep kicking, and since the opponent isn't attacking I was pretty much guaranteed to win each match since they couldn't get an attack in before I was able to damage them when the second round began. The only opponent to give me any trouble was the second to last boss because he has huge range with that pole weapon of his.
I think the biggest issue with your game is that you didn't set the hitboxes up to properly reflect 'beat 'em up' planes. What I mean by this is that someone can hit their opponent despite the opponent being 'further away' from them. To a small degree this is okay because otherwise it would be too hard to actually hit your opponent, but a lot of the time blows were connecting that visually didn't make any sense whatsoever.