the thing is that sometimes they spawn on top of you and take away some of your hearts without even being visible for a single frame. for difficulty increase I would have more of them spawn, it encourages good aim and multi kills.
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you can do this using clones. Also, I broke your game. when you take damage from a zombie it disappears forever, if you group them up by running circles around them, you can destroy them all and only take 1 heart, making there be no more zombies. (in my previous post I assumed you were already using clones)