This description here seems to be misleading some people, but all you need for this is five consecutive one-hit kills with a melee weapon. I got this with the sickle you earn in an early side-quest, while using Analog controls. With the analog control scheme, it is actually much easier to pull off a head slice with the right-to-left motion of the thumb stick. Let zombies charge you as their momentum seems to aid the decapitation. A good spot for this is while defending the gas station during chapter two. About 6-8 zombies will charge you when you open the garage door.
Another method is to join a friend toward the beginning of a new game with your high level character. The lower level zombies will easily die in a single shot.