Since my last pot i have obtained the acheivement. If you are stuck, but absolutely certain youve done all the missions = try playing the asphaar mission from the beginning, choose the castle path first, and once you have got to the end and completed the final mission(forest mission). go back to the first mission then choose the fort wayandeek path.
I got my achievement by doing that - even though i know i had played them all before on every charachter. Seems to me like they had to be played in order so that it would unlock.
~ Guitar Pair By Me =) ~