If you don't mind, here are the aspects that could be improved (I'm sure you've already noticed them, but I'm sharing them with you just in case):
The enemies at the beginning feel very silly; they can see you but don't approach, and when they do, sometimes they get stuck in front of a column.
It's also unclear when they're close enough to you; it's like a camera or perspective glitch. They feel like they're several meters away, but suddenly they get very close too fast. I don't know how to explain it.
Finally, the ability to jump while firing the shotgun is too easy. In the last level, even firing almost straight ahead allows you to stay in the air and move forward. This would be better if it only happened when firing at the ground.