Nice job! I had fun with this one, despite its simplicity. It has a similar gameplay loop to Root Beer Tapper, actually. One element you could take from that game is making it so enemies don't fall back down unless you really overload them with bullets. For example, in Root Beer Tapper you can serve a customer but they only get pushed back a bit, not always all the way off the bar. If the aliens shed some of the bullets off of them over time it would cause the player to either fill them with bullets or juggle which ones they're shooting at.
Just some rambling thoughts I had while playing :)