Bah, I mastered that game like a year ago. CrystalCrown and I were pretty much Tower Defense Masters back in the day... DaVe too. (but mostly CC and me)
This is so simple! Just create a huge maze with mainly pellet towers sometimes with some swarm towers, preferably at the end, where the Flying guys leave the screen. Also build lots of dart and squirt towers. This tactic works best for me =]
I can beat the game on any mode except hard, but this "100 Levels" challenge is kicking my butt. The enemies just become so strong that even the highest upgraded towers don't do much damage to them...and I've only lasted until level 40 so far!
Those spawn enemies are a real nuisance. You kill them, and they split into smaller enemies, you kill those, and they split into even smaller enemies, and so on. When you have about 20 spawns coming at you things get really dicey.