Objective: Survive for 11 mins 40 secs
With the right fire power on the top horizontal line you can make short work of the attacking enemies.
Having two fully upgraded laser towers blasting down the line will destroy just about all the enemies coming at you.
I build three scout towers in the early stages if the game while I upgraded my laser tower.
I also used slow towers to help keep the enemies in the firing line of the laser towers.