Location: Saw Mill (or anywhere really)
To get this trophy it's best to use the shotgun at close range. As a walker attacks you they will reach out to try and get a grab, pick out one single zombie (there is one free roaming to the left at the start of the Saw Mill location for example) preferably on their own and get their attention.
As they come to follow you let them try and grab you a couple of times to get use to where they reach their arms out to and which one they use most.
Then when you're ready shoot at their reaching/grabbing arm and shoot. It may take a couple of tries but once you shoot off their arm the trophy should pop.
Below is a video (X360 version) from my YouTube channel.