During the first area (at your camp just after the prologue) you will have to find some gas. Near the end of the area you will come to a slightly larger fence area with several zombies roaming around and your friend in the distance.
To get this trophy simply get the attention of several zombies and wait until they grab you, make sure you have four or more surrounding you. Once this is done you will have to move the stick to align you knife with their head and then press the corresponding button to kill them, do this until all zombies are dead.
Below is a video (X360 version) from my YouTube channel.