You must find half of the "treasure chests"; you will likely get this achievement while playing through the game. If not, there are incredibly helpful videos on Youtube, made by hsypersakura2009.
The videos are missing one: chapter 10, after verse 2, on top of the archway where there was an Alfheim portal in chapter 2.
Playlist:
http://www.youtube.com/view_play_list?p=9F20B689D5F3FEF5&...Related achievement: