It get easier once you get used to it. I’d like to see the power ups last longer, because they spawn in the middle of a huge cluster of guys that you have to lure away. Also, I may be missing the point but I suggest lowering the upgrade power, making them stack, and making them last from level to level. I think this would be a good idea if your going to include a better ai, because this ai is still hard to beat when there are too many little guys.
Edit: I noticed a glitch in one of the later levels (7 I think), where the enemies shots don’t move.