Here's another run with the same setup. I didn't get quite as far - a couple ants got wise to me at the end and slipped through along the bottom to avoid my ice cannon. That really screwed me. But I still made it to 86 without much difficulty.

Final score:



My setup (I actually was trying to add another sniper level 3 in the lower corner after this screenshot was taken, but I didn't get it finished... in my run to level 93 I had one more sniper, and my poison [top right] was upgraded to level 2, which I didn't bother with here):



The key is that I sold out my anthole attack to build an army around the pie. You can find a bunch of ways to kill a zillion ants at the anthole in levels 1-50, but the difference between that and level 80+ is finding a way to hang on to that last slice of pie. I do it by freezing the ant who grabs it and pouring as many sniper level 3 rounds into him as possible.