Tail lines and a vector from your player in the direction of the mouse cursor would go a long way --> check fitts law, your mind is working much faster than the game allows you to react. One of the arts of game design is finding the right balance between challenging and impossible.
EDIT: Or instead of a trail line, have a fading dot from where each enemy is created since you can still aim for that spot. There are other things you can do to make it more fun but keeping a fading dot could do a lot. Also you managed to spawn an enemy directly in front of where I was and I didn’t even see it - maybe some sort of glow effect around their spawn points.
The shop provides for a cheat opportunity, simply enter the shop and you have just paused the game!
EDIT: Oh it has trails, but the autofire is fairly weak in that it can provide a disadvantage. But this game is terribly addictive
EDIT: Ok, the trails need to clear themselves quicker, a fade out will do fine. Maybe a bomb function which you can do every 10 seconds that kills anything within a 30-50 pixel radius for a period of 100ms.