Cool game! I liked the variety of monsters each with different abilities. 

I wasn't exactly sure of how to get past the big shop thingy when you try to get the key, as it one shot me every time. I upgraded my ammo and health a bit but it didn't help. I think visual indicators of how much damage I took from it with damage numbers or maybe health bars on it/the enemies would help to know what is happening. I also never killed the shooty bullets ghost cus it's bullets would block mine and it seemed to have a lot of health, so some sort of indicator of how much progress I was making would help. Also when the things come from offscreen it might be useful to have a danger indicator so the player doesn't get hit immediately.

Pretty fun game for something so simple. 

Could have used some more fun upgrades though. The ammo upgrade seems to be completely useless due to how your ammo will always replenish at the pace of ~2 per second.