Starting out: Make sure the enemy only has one exit, put machine gun towers along the exit path. They receive maximum damage as they pass through the exit.

Boss 2: I forgot to take a picture at the first boss, but I need to mention that you should extend your wall-maze out a bit and upgrade the machine gun towers in order to kill the first boss. For the second boss, I had a flame thrower tower, a plasma turret, and some upgraded machine gun towers, it didn’t stand a chance. Also a missile pod for air targets.

Boss 4: By this time, I had a ground laser cannon, an air laser cannon, and an upgraded air missile pod. I also upgraded my ground plasma quite a bit.

Wave 65: Its pretty much the same setup except I upgraded more and sold a machine gun turret to replace it with a plasma turret.

Wave 80: I bought a heavy cannon for AOE damage at this point and upgraded my other stuff. Notice that I also wrapped my maze around. This ensures maximum damage at minimum cost. Looking back at the later waves, I should have bought more heavy cannons because the pathway got really congested after a while.

Wave 100: Well, this game actually goes on forever, so I stopped building around wave 95 or so and creeps got through, but I bet I could go at least for 10-20 more waves if I kept building. Or just keep wrapping my maze around. It works pretty well.

Good luck!