You'll find Trollshaws between Weathertop and Rivendell; it's the small area in which Arwen raised the water from the stream on the ringwraiths following the events of the 'Weathertop' chapter.
To get this achievement press and hold and select the fifth circle going clockwise. From here you can select ANY character. Select either Bilbo Baggins or Bilbo Baggins (Old). Use whichever one you prefer or have unlocked, as both will work for the achievement.
Fast travel to Weathertop, and begins to walk towards Rivendell. Use your map and/or set a destination if you're unsure of how to get there. Upon entering Trollshaws, the achievement will unlock. You can also do the opposite and come from Rivendell and go towards Weathertop if you wish.