I can't get past the third challenge level. I've made a spreadsheet of all possible upgrades to damage and rate and it seems that in the price range I fail at, the best tower I can build is the Rocket tower with 40000 damage and 135 rate (cost $6715), but even that quickly loses to wave 42 of the challenge.
I get really close to being able to upgrade it to 60000 damage (MAX) but not quite.
My other idea would be maybe a combo between two towers would be more effective. However it seems impossible to afford two maxed out towers - the cheapest is the Tazer for $3360, but that has such short range I lose whenever I try it.
Any ideas?