Fun fact; you can beat practically the whole game just with the basic red units. Just buy a whole bunch of them, surround the large groups of enemies, click "play", click "play" again, readjust your units to surround them, and rinse and repeat. Start circling smaller units (Like those green snipers, if you can see them) abit later.
I learned this shortly after seeing how effective level 5's circle formation was at obliterating anything within it. If it's not level 5, then i'm sorry, but as I was writing this i tried getting level 5 up, only to find the levels would no longer open .-. Weird glitch, but at least i beat the whole game anyway.
Btw, please don't take my comment as a negative; This game was great :D