A straightforward achievement but a very important thing to note: you need to liberate 2 outposts without being detected on your first play through of one. You cannot get this skill unlocked if you don't which will keep you from getting the acquiring all skills achievement.. Do this first into the game as its the easiest when there is only one alarm and a few guards.
Once you have that done, go have some fun and grab a buzzer, grenade launcher, and an rpg and reign hell.