I felt the better tactic was to get a good core of factories in the center around the main base, then keep bolstering the death ball with more and more with the armored thingies. I experienced that also placing them out as far as possible gave each "turret" more time to deal with a smaller amount of enemies before blowing up. At this point in the game, the camera was skipping around and the main objective was no longer in the middle of the screen :p