You will get this achievement for completing 20 sidequests in the game.
You can get sidequests from characters you meet in different chapters. You can also get personal quests from your companions which also count as sidequests.
Always go around your current mainbase and talk to everybody and offer them your help. The quests are straight forward - find someone, find something, fight something, collect something, ...)
After starting a sidequets you will get a marker on your map (sidequests are marked with "2" - main quests with "1"). Simply follow the marker to complete your objectives.
Those sidequests will earn you a good amount of XP - so it's a good way to level up your character.
The achievement pops after finishing your 20th sidequest - for me this unlocked around the beginning of Chapter 3.
If you have any info on how to improve this solution feel free to comment!