After locating a Night Chest all you have to do is hold down the start button (3 lines) on the controller to have Fenyx rest (meditation from Assassins creed) and move time to night.
Battle the enemies guarding the chest then open the chest for the Achievement.
Edit Solution: There is one Night Chest in the Clashing Rocks area. It can be found on one of the many islands, to the southwest of the Clashing Rocks mainland. This one is easy to locate fast but need a good amount of stamina to reach it early on.