Group Hug Achievement
Kill 4 Walkers in a Single Grapple Sequence
Simply get grabbed by a large group of walkers and then you will be passed around from one walker to the next. All you need to do is kill 4 of them in one sequence and you will unlock this achievement. Remember that you need to aim for the head and press to kill them.
Here's a quick video to provide some assistance:
Thanks to warrenpaul for the video.