Really, really nice art, and so much of it in just 10 days! Having the sprite be lit a bit in the direction of your gun was a really nice touch.
My one suggestion would be to add a bit more juice (links to a talk that discusses this). For example, hitting the boss results in no change except for his health bar: that's a perfect place to just make him or the screen shake a bit. Especially because of how good the art and gameplay already are, I bet adding just a couple well-placed shakes and (programmatic) particles on impacts would make this game feel super good with relatively low effort.
Super minor criticism though, it really was a great game!