At the very end of the DLC, when you get to the landing pad, you'll be confronted with Mejia and the Fixer. In order to get this achievement you need to save the Fixer's life (there are likely mulitple ways to do this...I ended up in a fight with Mejia and had to tranq him before he killed the Fixer). Once you do this, talk to the Fixer to bring up the option of choosing what to do with him and unlocking the achievement.