You can't just kill a zombie once they are down, you have to attack them in a way that 'kinda kills' them and then you have to wait for them to start the revive process and then kill 'em.
One way to do this is to kick zombies down stairs and as they revive, pop 'em - I used a Webley. Simply kicking a zombie knocks them down, it does not do the same damage as kicking 'em down stairs, I guess!!
You know when you get a correct kill because the kill type is "Resurrection Kill" and the cheevo counter will go up.
I ground this out heading to the rail yard on the Labyrinth of Death level. There're loads of stairs around here; the steps in the video below are just as you see the train, you can climb up stairs over tracks and down again. Zombies swarm at you in this level.
See the video.
http://1drv.ms/1Abl6ed