Complete any Outcast Rescue Mission to unlock this achievement.
You gain the ability to do Outcast Missions after you meet Hirgon of Tarnost, the red-headed ranger, in the main missions.
The Outcast missions are identified by a White Fist with a White/Crest Border surrounding it. If it's only a white fist than that is the side quest giver and the achievement won't pop up if you only help them.
In the video below I did the Mission Shattered to get this achievement.