Under options you can change your control scheme from Digital to Analog; this makes you eligible to begin racking up kills for this achievement. Technically, you don't have to kill enemies in this mode expressly -using- the analog fighting--as delivering kicks and other means of zombie death still count.
However, while at first you might find yourself driven a bit nuts by this control scheme, I highly recommend giving it a shot as it may well grow on you; the biggest advantage is when you go into 'attack mode' with the left trigger, you can then 'lock on' to specific body parts on enemies.
This is a tremendous boon against tougher enemies such as Thugs--for afterward you can target their arms to break with bludgeoning weapons or cut off with slashing weapons, making them -much- easier to handle afterward!
Control note: In Analog fight mode, the right trigger takes you into 'throw mode' and the left trigger takes you into 'fight mode'; when you pull the left trigger, you will essentially lock on to whatever body part is in your cross-hairs, and this is how you can try to specifically behead a zombie with an edge weapon or remove an arm, etc. Moving the right analog stick while in 'fight mode' will position your weapon accordingly--you then 'flick' the stick to deliver the blows.