Whilst this can theoretically be unlocked during any job my recommendation is to start an offline Meltdown. This stats you on a roof overlooking the Murkywater Warehouse. As long as you don't leave the roof enemy spawns will be limited to Helicopters dropping off squads of four, the occasional ground spawn and snipers.
The Lebensauger only holds ten rounds and such this achievement will require a double kill at the very least. Take out your first nine enemies preferably focusing on snipers and enemies dropping from the choppers then simply wait for two or more enemies at ground level to group up in a position where you can line up a shot.