As Crimson Drifter mentioned, this achievement is easily unlocked during a normal playthrough the game. This is one of the achievements that just comes naturally.
A good tip is to assasinate one of the guards, when trying to rescue a civilian. This way, one of the guards will be eliminated immediately.
When running on the rooftops look out for guards as you can easily assasinate them, while travelling towards f.x. a mission location.
Simply just assasinate a guard when you get the chance and this achievement wiill be unlocked in no time.