Repeatedly hit a Zed in melee combat until goes down flat or it is on its knees. Get near, then press Y PC to perform an execution. You can use this tactic to take down large numbers of Zeds quickly to reach the total needed to earn the achievement. As soon as one is executed, and start on the next. The Zeds still on their feet cannot attack you while the animation is dispalyed. You can pick apart larger groups one by one without getting surprised by Zeds that get up again.