This achievement is unlocked at the beginning portion of the 'Countdown' mission, where you are rappelling down the side of the building. There are 3 hostage encounters -- each floor will have an increasing number of enemies and hostages:
* 55th floor has 2 enemies
* Next floor has 4 enemies (and a checkpoint will trigger after this floor)
* 51st floor has 5 enemies
You'll have to kill all 11 enemies without the enemies firing their weapons, which means, you'll have to kill them all without being detected. I recommend staying in the "Australian rappel" position, and walking out towards the sides to pick them off, where you'll be less visible.
At each floor, once you begin shooting, you will have a short amount of time (~30 seconds or less) before the enemies give up on trying to find you and simply shoot the hostages, which obviously causes the mission to fail.
The enemies will generally walk towards the window once you start shooting so anticipate their movement.