This achievement is for opening the loot chest in the old town of Fyrestone which is now called Arid Nexus - Badlands
Use any travel station to travel to Arid Nexus - Badlands and once there follow the path through the town until you come to an opening next to a large highway (This is where you fought the large robot) On the right there is a building, the loot chest is on top of this building.
You need to make your way to the back of the building and jump
onto the hand rail by the steps, then onto the roof next to it and then back to the other side on top of the small vent like thing protruding. Then jump onto the roof and look for the red loot chest like the one in the picture and hold
to open it and the achievement is yours.
Here's a video (Yup I forgot what to do at first