Could use some indicator of how long till the stage ends - a completion bar, amount of enemies left, how long left, idk, something so that it's more apparent that there's an end in sight.
I got into the habit of just shooting at the places where the enemies come on screen from, this caused them to die offscreen, and all their lovely energy left uncollectable by me... (didn't realise this for a couple levels) if this is intentional, a warning or such would be nice, if not then please could you prevent it being possible to kill offscreen?
As others have already said, massive lag when there's multiple explosions on screen, smaller ones, or something else to combat this would be good.
I don't think you already do this (because it plain doesn't say anything in game, other than kill them running towards you, pick up glowy things) but maybe some reward, or at least tracker for accuracy? Rather than just spraying bullets everywhere?
In line with the above, a background would be nice, rather than just "kill ze humans, they be stealing our stuffs". And also a "how to" that's accessible outside of the start of the first level.
Graphically it's awesome, but the mechanics (as said above) could be worked on a bit to polish it off.