Contrary to what other achievement sites tell you, you cannot simply run around with your level high, throwing the jet-troopers from the roof.
You must knock an enemy (one without a jet-pack, of course) off of a rooftop to their death. This is easily done during story missions involving rooftop battles against the Enclave; thunderclap them near the edge to send them to their peril.
Does not need to be five at one time.