Crisp art, nice controls, good gameplay loop, smooth scene transitions, appropriate audio. This is a really tight package, and feels relatively complete as a little mini game. Reminds me of something that could be featured on Newgrounds back in the day.
My only comment is, it's a cool idea that 2 characters spawn for every 1 you kill, but I wish that were more obvious within the game itself. Currently it just feels like you get overwhelmed for no reason. I'm not sure how you'd accomplish that though.
Really well done!