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. You can have to continue through the area and kill all the enemies before the achievement unlocks. The game automatically saves after your decision, so you will have to get the achievement for the other decision using chapter select, or another playthrough.