For those who said it's too difficult... I beat it on the first try. You must just suck at shooters. Upgrade cannon once first, then upgrade units once, then get triple shot. Then upgrade cannon again. Then get missiles, then get secondary cannons. Then upgrade cannon til max. Then get your units again, then max your engine and shield once or twice. Easy, but tedious.
For stage 3-1 I had a rough time until I spammed the hell outta maxed mice, with one turtle in there early and one later on. You may get hit by the zombie, and have to run back and use an AOE mace to get the ghosts, but keep spamming mice. That got me the 3 star win.
Anyone else notice that the red platforms don't count anymore (problem with programming?) so you only have to remove the red pieces with faces?
I'm sure y'all did, but I didn't see a comment about it yet...