There is a BOUNTY HUNT mission in WESTMINSTER just west (left) of BIG BEN (the giant clock tower) and south of The Foreign Office. Both of these landmarks are marked on your map if you've synchronized the viewpoint for the area. The mission is to kidnap Wade Lynton.
If you access this area from the rooftops you'll see three enemies standing under some hanging barrels. Toss a knife at them and you've got your achievement. It's as simple as that.
This may require the Gang Upgrade called Delivery Service, which is in the second tier under the Insider section. This upgrade makes it to where there are more hanging barrels around London for you to crush unsuspecting enemies with. I already had this upgrade when I found this area to knock the achievement out.
Here is the clip of me doing it.