I did 4 warlocks and a paladin up to ~20 and it was pretty easy. Created macros to set the focus for each of them and before a fight I would have each of them setup with a different focus. Then I would target one and they would all DoT it and all the void walkers would attack their respective focus's. Then I would go through DoTting up each of them. The paladin was just there to heal the voidwalkers when they got low.