Nice shooter! I liked the music very much. The progression system feels nice, although I'm not sure how much the enemies get stronger. I was taken aback by how much damage they deal! I felt confident with 70HP, haven't being hit once, then I died very quickly when assaulted by a few big zombies.
I survived for 03:29. I even saw and defeated a boss, which felt great! The game can get a bit repetitive though, as it is not much challenging for a long period, and suddenly it becomes intense in a few seconds and kill the player instantly. I don't know if this is so much a balance issue, maybe it's also that the mechanics is mostly "shoot, run, and wait until the next upgrade"? Maybe more challenging enemies right at the beginning, that demand to react quickly, could break this routine.
Still, I had fun playing it and reaching the score I did. Good job!