To earn this achievement you need to roll a 1 or 20 on a Dice Chest.
Dice Chests are unique to areas located in Tiny Tina's Assault on Dragon Keep DLC. If you've yet to begin Tiny Tina's DLC it can be accessed by visiting Unassuming Docks at a fast travel station.
Dice Chests are easily recognizable, they simply look like a normal red loot chest, but with 2 colored 20-sided dice on top. Eridium is required to open these chests
, and the player can choose to spend 1E to roll one die, or 5E to roll both. (The only difference here is that the higher number you roll, the better the loot you get).
Earning this achievement is luck based, I made it through the whole DLC without getting it on my first playthrough.
Lower level solo friendly method:
Another, and probably better location, requires you to have the ability to defeat the raid boss dragons. On Normal Mode these are around level 38 for a solo level 50 character: