It's a nice prototype that you had with your own art style, congrats !
Improvements:
I found it somewhat repetitive and had a hard time going through all 200 kills since it got boring rather quickly to be perfectly honest.
A quick improvement would first to add some background music to make the killing a bit more enjoyable, it's a nice trick to make players execute redundant tasks withour realising that time flies.
On that note, it would also be nice to play the attack sound with a random pitch, as having the same sound playing in your ears 200 times (literally) can be tiresome.
What went right:
The game is incredibly straight forward with simple controls, and the UI is great by it's simplicity. You know exactly which ability does what and when you can use them, that's no easy task.