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.
Also you can do it with the fire axe get around them and let thier back face you and start swinging, once they lose thier leg the walker will fall to its one knee giving you time to swing for the head and boom you got it good luck!