During Chapter 7, you will come to a point where three soldiers are beating up Goul. You have to decide whether to let him die and sneak out of the place, or step in and shoot the guards, causing a big fire fight.
For this achievement, you are going to want to save him and shoot the guards before they kill him. This you will get this achievement once the fire fight is over and you have moved on to the next area. The game will checkpoint so you will need to replay chapter for the opposite decision