The achievement unlocks when you conclude chapter 3 and tell your faction leader you are ready to progress. It is story related and cannot be missed.
When you defeat the monstrous chapter 3 boss and resolve its aftermath, you can proceed directly to your faction leader to conclude the current quest and move on to the final segment of the game. Nevertheless, you may have depleted your consumable inventory with the intense fights at the end of chapter 3. Accordingly, it may be prudent to first go to the village and replenish your stocks. Not only does the final section of the game start with combat (following some cut scenes) but there will be no more merchants available from this point on.
Once you are satisfied you have completed all the side quests you set out to do and optimized your inventory, tell your faction leader you are ready. You will proceed to the final section of the game and the achievement will unlock.