This was fun, but really tricky. I think having the health leftover to use as shop points would really help here, especially because items are one time use. I also noticed that some of the bosses (who stand still when attacking) are pretty easy to push into the corner with the knockback from the gun. This makes cheesing at least (5 of them that I noticed) pretty easy to cheese like this.
Otherwise, this is an incredible game. All the attacks are unique enough to feel different, but similar enough to be able to react predictably. All the art fits together really well, and the controls are really snappy and feel very natural to use. Great job!