Hello!
I liked the billboard art style! The weapon switching mechanic was hard to understand, and it was a bit hard to see. Reducing the movement smoothing might make the controls feel more responsive. The giant boss was fun to fight. I wasn't sure what the weapon icon at the top of the doors meant, but assumed they told what weapon I would get by traveling through that door.
The music got a bit annoying. Maybe a longer loopable track that continuously goes on in between the levels? In the win/lose screen, it seemed as if the game was still running in the background. But other than that, nice arcade feeling!