I really think your best option here is to have your UH DK kiting the adds by himself, with one of the pally healers healing him. Unholy aura gives the speed boost, and just have him in frost presence. Stack your ranged all together, so all the zombies go to one place and a simple DnD on a shorter cooldown as he is unholy will pick them all up. This is a brilliant setup.
When decimate is close, have your kiter pick up the last zombies and take them to the back of the room. Then using PS will trigger unholy ground during decimate and hae you dps AoE. I can't see what trouble you could possible be having.