When the enemy pulls back to make a punch, you press the block key. Sometimes you need to press it two or three times. Then you press one of the two attack keys (which one doesn't really matter) a few times. You repeat this until the enemy is dead, then walk forward on the bridge a little, meet another enemy and do it again.
I'm not just telling you the best strategy - I'm telling the ONLY strategy. The game doesn't even let you attack until after you've blocked first. There's literally zero variation, no strategy, no combos; just block, block, attack, attack, attack. It's a bit like Punch-Out, but without the dodging, stamina, unique enemy attacks, or any of the other things that made Punch-Out fun.
After doing the same thing a dozen or so times, I stopped playing. I can't recommend this game to anyone, even if you got it for cheap in a bundle.