Very good concept. I like the fast-paced gameplay with the idea: kill or be killed!
It is very satisfying to see those particles when we destroy an enemy, good idea to put some of them in heights so targetting with the mouse is actually useful.
A tip to eliminate enemies without getting hit, is to hit toward the right part of the screen even before seeing any enemies because the bullets can still hit them outside of the view of the camera.
Also, good choice of music, it's a good entry!