Also, the penalty for losing a level is a little high. Many times I'm retrying levels three or for times and nothing is getting improved. It may require a little re-balancing in the rewards but it would help a bunch.
The acceleration is way to slow and that makes dodging things a little difficult. As for everything else, what everybody else said (magnet, boss health, etc.) are good ideas.
Oh and the end game becomes a tad bit easy. The final weapon combined with the final turrets is an almost overpowered combo. Your gun takes down all the small guys and the turrets will take care of all the ones heading straight forwards. For a few levels I just left my mouse in one place and it was over.
It gets pretty easy to get to the end. Just upgrade the turrets and EMP whenever you can, always making sure your health is above 50%. I finished in 16 waves but I probably could have done better as my technique started to improve at the end. I'd recommend more enemy types, the option to place turrets where you want them, and the machines attacking your upgrades so you can't be too complacent.
Good game.
You have to use the gust of wind and then use a lightning bolt on one of the enemies to get the killing blow achievement. And it has to kill them of course.
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