Vulcan large/high. 15.9m emitter open to surface, stream of drones from start, 12 phantoms, no gravity. The screenshot is my best holding action at 39 seconds. (got 2 crystals, built blasters and managed to hold creeper and drones off until power ran out.)
Actually I can't get a start on medium or high complexity ANY size on this one. Low is possible - has gravity and no drones.