Easy way to grind this Challenge is to make your way to the Medical Storage Save Room (Mansion Map 1F West Side) and avoid the Zombie on the Stairs.
Enter the Save Room and Save your Game at the Typewriter.
Exit the Save Room and move against the wall behind the dead Zombie on the floor.
Now wait for the Zombie from the staircase to walk over his dead buddy and put a round in his chest and one in his leg.
Walk up to the two and press
to open you invetory and then press
on the Fuel Canteen to set them both on fire.
Rinse & Repeat 49 more times and your Achiev... Challenge should unlock
I will post a video soon!!!!