You will get this trophy easily during your first playthrough. The earliest opportunity you can unlock this though is right after the very first dining room scene with Barry. Check the door to your right and enter it. In the hallway make your way to the left and trigger a very well-known cutscene:
Jill - RECOMMENDED-: You start the game with 15 bullets for your pistol. With Jill it took me exactly
9-11 shots to the body (no headshots) to take the zombie down on
NORMAL difficulty (obviously less on lower difficulties).
Chris -NOT RECOMMENDED-: Well since he got no pistol and only a knife equipped at the beginning it can make things a little bit harder. It took me exactly
14 slashes to the body to get the this fool down on NORMAL difficulty (obviously less on lower difficulties).
The trophy unlocks as soon as the "music stops" and the zombie is down for good.
Let me know if I should add anything to improve this solution. Happy zombie killing!
