In order to get this achievement you will require to pick up the treasure item the mushroom crown it can be found on the mission black rider once you reach the section
where the rider is on the bridge and you scare him away walk up to the collapsed bridge and break it behind it you will need some one with the rope skill or have made the item at the blacksmith.
Pull down the glowing hook and a tracking item will drop use Aragorn in order to use it follow it to the second track item from there it will lead you to the crown when you pick it up it will go to the inventory and should pop if still using Aragorn if not then select him from the top row (variations of Aragorn royal armor or gondor armor do not pop the achievement the crown can be selected from the treasure chest).
Attached a video by JuicyBoxMan for a visual guide if needed.