This is a secret easter egg achievement that you unlock in Vaulthalla, which is accessed at the end of the DLC. On your way, during, and after the final boss in the 60 second loot room are hidden symbols you have to shoot. A secret room then opens and you have to find even more of these symbols, revealing ANOTHER secret loot room. That's where you get the achievement.
Since these would be almost impossible to describe, I'll link the video guide that helped me, courtesy of DPJ. It covers everything in less than 5 minutes. Enjoy!Update
: The only things that I'd note here that you can do differently from the video (thanks to comments below) is you can run out of time in the loot room and still hit the switches. Also, for the switch where you have to smash the floor, you need the guardian rank perk “Cmon and Slam” to replicate how he does it in the video. If you don’t, jump off of the ledge above it where the exit portal is.