I think you have to click on the actual enemy to shoot. Gave me quite a headache for the first run I thought I just had to get the angle right. The background art is very nice, but the rest of it is kind of a weird mix of low and medium-high res art. The difficulty ramp at the first speed up is pretty intense too. Maybe a more gradual speed up would be nice or smaller increments. As it is, it's very hard (and I want to say it's a little buggy, because I'm sure I was clicking them) at the higher speed. I didn't make it very far past the first speed up, so I don't know if there are more levels. I can't imagine there's another speed up it would be way too fast if there is. Music is nice and fitting for the theme, and I really like the pixel backdrop. Looks nice.
Edit: I would add that the jumping is a bit floaty, but I'm not sure if jumping really is needed for anything anyway. Is it?