Allied yourself with the smugglers upon arriving in Kirkwall.
This is missable. When you arrive in Kirkwall, you will meet up with your uncle, Gamlen. After you are done talking with him, SAVE your game. Seek out either Anthenril to the north to ally with the smugglers or Meeran to the southeast to ally with the mercenaries. After completing the specific task, the achievement will unlock. Simply reload your save and complete the quest for the other faction.