i enjoyed dodging bullets & clicking off baddies in this game
however, the game progression isn't quite clear in my opinion, my suggestion is either make the game displays which level you're on, and/or some kind of indicator whether you finished a level or lose the game
in my opinion, each one of the enemy character could be further emphasized based on their behavior
for instance, the green one could be some kind of homing missile, the red one could be some kind of bull that charges at you, etc
also, enemy interacting/killing off eachother could provide some interesting gameplay, so you can make the green one obliterates the blue one when they collide eachother, but when the green collide with the red one, they get stronger, for example