Right I have the Quest Chian for this final Achievement now. I thoughtI'd tell everyone how I got to it as it might help others in the future.
Now I can't remember the name of the Quest, but I would recommend you do all the Side Quests in the 'Gallia' City as well as the extra DLC quests.
There is a Side quest where you have to seek out a Madusa that has caused trouble. This asks the player to go to 'Skellheim' (I think :S) and once there you are given the choice to either Kill her like asked or take her word that she won't cause any trouble again. (I chose to take her word and lie that I killed her, but I don't think it matters which choice you make).
Once I did this Side Quest, the quest from Drong appeared which starts the Quest Chain for the Final Achievement.
Here is a map to help find the places I have mentioned: http://faqsmedia.ign.com/faqs/image/..._world_map.jpg
I hope this helps