MUCH easier solution:
Make sure you have the wheel, voodoo, and inventory mutators activated.
Go around, collect four zombies by licking them with
to put them into your inventory chest.
Stop time with
whilst the wheel mutator is active. Then open up the inventory with your
, then press
on all four zombies that are in your inventory so they all get spawned in the same place. Press
to move out of the inventory menu, and then move a step or two away from the zombies. Press
again, which will both restart time, and fire a voodoo flame ball thanks to the voodoo mutator.
This should hit the four zombies you've just placed in the same spot, and reward you the achievement with minimal effort!