Crash Course in Brain Surgery Achievement
Crash Course in Brain Surgery25
Kill 100 Walkers while Grappling
This will most likely come with natural progression of the game. When there are a large group of walkers near you they will typically take one swing and then all attempt to grapple you. During these sequences you must force your to the head of the grappling walker and then quickly press in order to kill it.
Remember that these stack through deaths, restarted levels, and playthroughs.