Very nice entry! I think the take of the Tetris styled endless puzzle games works really well for this limitation.
I found on easy the game was basically play forever, I even had to step away for a bit and came back, was still alive and recovered with ease. I find in infinite games like this its good to have some sort of scaling so that you cant play forever. That being said on easy I played up to 20k points before just letting myself die. IIrc how tetris does this is as you hit certain scores you go up in difficulty, and the difficulty just determines where you start. That would be very welcome here I think!
On hard I think the game does everything right (tho I do just enjoy hard gameplay :3) the constant stream is fast enough that keeping up is going to be hard and it does keep escalating naturally as a result of the system. Good stuff! got up to 4K on that one.
The controls where great, felt good and the aiming felt very accurate. The visuals were clear and it was very easy to figure out what was going on. The tutorial screen communicated the important details as well clearly and quickly (and its great that I can reopen it!). Only thing I can mention is that once or twice my mouse would leave the window and I would not fire, not a big deal tho once I adapted.
Great entry!