I like the art in this game. It's simple but definitely gives a feeling of urgency! I got a B in the end :)
Like @LesPaulLord I thought the game might be broken, so it'd be nice to either add a sound effect for shooting or indicate it visually. Also 30 HP seems like a lot. I'd recommend moving the gremlins more instead of having such a high HP.
Overall, I liked how consistent the art style was.