MUCH easier solution:
Make sure you have the Wheel Goat, Double Jump, Inventory, and Prototype mutators activated. If you haven't got Prototype, you can get it atop 'Golden Tower', where the scientist is doing experiments. Stand on the bed in the middle of the roof, and you'll get the Prototype mutator.
Go around, collect five 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 five 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. Jump over the five motionless zombies, press
again, which will both restart time, and do your bullet attack. You don't have to kill the five zombies, just hit them.
This should hit the five zombies you've just placed in the same spot, and reward you the achievement with minimal effort!