The final step of the Mannequin side EE can be a bit of a pain to clear depending what round you do it on. Here I was on round 29 and managed to clear it easily by popping an energy mine turret in the room. It turns this step into a breeze.
I want to try this same thing on the defense steps in the boss fight. I have a feeling it will clear the defenses if you train zombies in the middle of the map while the energy turrets clear the 3 defense areas one at a time.