As with the Master Marksman achievement, the Devil's Pit near Velen is a good place to get this, although this achievement is substantially easier than the other.
To get Humpty Dumpty try to get just one or two guys to follow you up to the top of the platform inside the pit, then use your block to maneuver one near the edge. Once he's there, blast him with Aard to knock over the ledge and (hopefully) kill him. Repeat with a few more guys. Once you've run out of convenient bandits, use the fast travel sign outside the pit to go somewhere else and immediately return to respawn all of the bandits.