When you're working on subverting the main missions, you'll almost always have to help your buddy out of a firefight at the end. Almost every mission that I did resulted in that buddy bring wounded. When this happens, you'll hear your buddy calling out for help. Look around for the plume of blue smoke and follow that to find your buddy on the ground. Simply choose to shoot them and you'll unlock this achievement. If you want to keep your buddies alive for as long as possible, you can simply reload your most recent save and replay back up to that point and choose to save them once you've unlocked the achievement.