I personally love high-contrast mode. I can't do boss fights on Heavenly without it, since it makes my laptop chug. Though the game still claims it's on 60fps...
Ick, #9 got me because I didn't think you'd have to restate your deduction. (In other words, Line 5 = last box of Line 4 on problem 9, for anyone puzzled about that.)
I think it's a bit lame that you can't get multiple Gold Skulls at once. For instance, despite getting over 540 on the second "challenge," I only got the first Gold Skull -- even though I got more than double the requirement for the third. Please consider unlocking all Gold Skulls you would have earned instead of forcing the player to play the same monotonous game at least thrice...
There was one thing that was really quite annoying, and the reason for my loss several times at 100k+ scores. Sometimes, when you hit a Force or Chain Force powerup, you'll be lead into an area with no escape -- or alternatively, a rival neuron will immediately spawn on the only escape path. This really can be quite frustrating; it'd be nice if this was changed! (:
For those stuck on day 17: the key is to take out the panzerschreckers as a priority target; use your mortars and sniper on them, and your bazooka on the tanks. (Leave the MG for whatever's the most ahead at the time.)
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++MG may shoot multiple soldiers, so sometimes it's useful to wait until they get up.