There's a major difficulty issue in Zombie Massacre.
It mostly comes from not being able to move when there's a big wave of zombie and the low framerate some people get while playing the game.
you should make the collision box smaller in order to fix that, because 90% of the time people get stuck around corners not being able to move, and there's such a big jump in difficulty between Day 2 and Day 3 that it makes it barely playable for some people.
It makes the game very boring, tbh.
Oh btw, standing perfectly still on any level below the hard difficulty makes you nearly invincible.