*SPOILER FREE*
This achievement is obtained after you have completed the final story mission.You begin it by going to a man in Blackwater, who can be identified one the mini map as a purple question mark, as all side missions are started. Follow the instructions, and when you get to the final standoff, put a shot in the person's groin, then the rest in his head (worked for me). After that person is dead, the achievement will unlock.
Cheers!