Although it seems like the main mechanics is not too original, the execution is very good. I wish the game was more forgiving (maybe the character could take a couple of hits before dying) but it's fun and engaging nonetheless. Piercing bullets kinda contradict the theme by allowing to sort of control the horde of enemies xD Just kidding, I love it.
Btw, seems like a lot of games have enemies spawning right besides the player, and it's really nice that you managed to make them spawn off-screen.
Keep it up!