All you have to do is kill 75 guards with knives. A good idea is to also go for "The Hands of the Thief" Achievement, that's the one where you have to pickpocket 200 knives from thugs. If you do this later on in the game you can carry more knives and it makes it a whole lot easier. Just run along the rooftops killing everyone up there and sniping the patrolers on the ground. It take a while but you'll get there.