Have a Nice Trip! Achievement
Have a Nice Trip!20
Kill a Walker That Is Falling Due to Dismemberment
This isn't as hard as it may seem. The achievement is fairly lenient with the grace period between dismemberment and death giving you a few key seconds to get the kill.
Essentially what you need to do is blow a leg off a walker and while the walker is falling kill it. If you don't get this naturally in your play with the shotguns then definitely try to single out a walker and shoot the leg off first then go for the head.
Here's a video to give the visual assistance:
Thanks to warrenpaul for the video.