Looks cool! Its usually hard to give feedback without having played the game but I will try.
The explosion animations looks nice, but I think it needs to be bigger on the bigger enemies, so it covers the whole sprite and a little more around.
I don't think this is a hard rule, but when I think of level design for shooters, you want it to be things on the screen all the time so its a challenge for the player to shoot all of it. I doesn't need to be dangerous stuff, it can be boxes or just a lot of popcorn enemies, but on different sides of the screen. Just so the player never feels like they are waiting on the level to scroll to the next enemies.
The homing missiles looks epic, how did you make them?