How do you get the Stockpiler? It seems impossible to "complete eight waves without spending a cent". Do you use no towers or just use the ones given?
I got it the first time on level 32: Slow Em Down, but I've gotten a on a few others now. I get it after I've covered every available cell with towers and completely upgraded them. At that point I have nothing left to spend my money on, but I'll still be able to plow through a several more waves.