Right in the beginning of Chapter 9, rappel down and continue on. After the cutscene, will be surrounded by snipers. You can either follow their orders and shoot the civilian or you disobey and shoot the soldier.
For this achievement, you will need to open fire on the enemies. Do this by making your way behind the car on the right hand side, and shooting the snipers one by one. 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