I really like this game, and don't have any major complaints against it, in fact I don't have any "complaints" at all, but rather suggestions on what could be changed (I don't want to say fixed, because none of that is really broken, so yeah).
I would suggest to kind of change the animations of the explosion of the enemies, because I think it kind of looks a bit blurry or not as sharp as the rest of the games textures and sprites, I don't know if this is intentional because of the fact that it IS an explosion or not, but I personally would make the explosions a bit more crisp and diverse, so that they kind of blend in with the rest of the game (and not for instance have 3 of the same explosions next to each other when shooting down a big airborn enemy).
I also think it would make more sense if the second weapon, that you can obtain, would decrease slower (or entirely stop decreasing) if not fired rather than decreasing at the same pace at any given time, because now, I do not see a reason to ever stop shooting, even when getting the secondary weapon.
Finally, I would find it very good if the enemies would have some kind of weak spot to them (as an example, the head of the infantry troops would be a nice crit-spot), so that the player has the urge to hit them in that specific spot to hurt them more than normally (maybe thats already in the game and I just didn't play it as far as that point ;) )
Anyway, solid good and fun game!
4/5