Chapter 13, you will come to a part where you must run and find your other partner to help him (this is after you rejoin with your first partner and fight on a little bit) Once you find him there will be a cut scene. After the cut scene, you have a choice to either fire into the air and let the civilians run away, or you can open fire on the civilians.
For this achievement, shoot your gun up into the air let the civilians run away. Proceed on a bit, making sure not to kill anyone, and the achievement will pop. You can reload checkpoint and get the other decision
Video has both achievement decisions