This Achievement is awarded for opening 290 treasure chests. This accounts for 95% of the chests in the game. Katz chests and Bonus Dungeon chests do not count towards this achievement. Be sure to travel and explore areas, Some Chests do respawn however only the first time opened will count towards the achievement. I find it best to collect all chests the first time through before moving on the to the next area to make things easier. If you do not have room for the item in the chest, abandoning the item will open the chest and have it count towards the achievement.
Defeating the Harbinger of Death Code Red Daemon will give you a Ventite that will show you how many unopened chests for the area on your mini map. You can also check your progress in Players Records in the menu