Very well done, really liked this one! The idea is good and the execution is too. Nice use of particles and the parallax as well. The mechanic is quite well done too, since you combined movement and attack on a single input... nice work! It is a bit hard moving, like other people said, but that is part of the fun I believe.
Not sure where you could improve in my opinion... this game already is quite polished. Maybe changing some of the more "beep-like" sound effects (like the laser one, if I recall correctly) would make this game more atmospheric, I think. But this is just a minor suggestion!