When you are in a A.P.R. or U.F.L.L building they will give you a mission once you leave the building (if you accepted the mission) your best buddy will call you on your cell phone and tell you to meet him. Once you meet him he will give the mission a little twist so you can help him/her out. Now when looking at your map once you accept helping your buddy you will see a red maker, that is where to go if you are just doing the mission, and a blue marker, that is where to go if you are going to help your buddy. Just help your buddy subvert the mission once you are done with the mission you will get this achievement.