i would recommend doing the final-final level directly after the level cap is removed as all those smiths give you so much experience you level up (and so gain full health and focus) every few kills. this gives you a fairly good shot at completing it
its probably a problem with my keyboard not the game but i cannot press up + left + space at the same time it wont recognise the up command. it works fine with all the other direction combinations plus space just not up and left. makes level 22 pretty hard.
the physics of the game make it very satisfying to play but the delay between shots is a little bit too long and gets frustrating after a while. If possible you should make it so you can fire the next shot while the previous one is still calculating which balls it has hit 4/5
some sort of numerical gauge in the corner of the screen would be useful. the power bar is so sensitive that just a few millimeters means you overshoot what your aiming for. some sort of predictive tradjectory line would do the job too but that'd make it too easy. Maybe a "ghost" of your last shot to be a rough guide?