This is for stealth take-downs only. Killing an enemy with a silenced pistol doesnt count, it must be a melee takedown. You can do this quite easily in a lot of places, but one handy location for it is in chapter 4. There are a lot of enemy camps that contain a bunch of enemies for you to takedown. Mark the enemies and it will be a lot easier to make it through without getting spotted. It is best to make a manual save at certain points so if you screw up and get spotted or for some reason the stealth take down doesnt trigger, you don't have to start again from the beginning.
Stealth take-down 15 enemies in a row