While exploring through the maps in the Tiny Tina dlc, you'll eventually come across chests that have two dice on top of them. You are given two options: press x to roll one die, or press y to roll two dice. If you roll two dice, then you have to pay 5 eridium. When you roll the dice, a number should come up. You need to roll a 1 or a 20 for the achievement to unlock. It doesn't matter whether you roll one or two dice. When you roll the 1 or the 20, the achievement will pop. Also, if you roll a 1, then take a few steps back because bombs will pop out of the chest.
If you don't have it before fighting the raid boss then after killing the raid boss is literally a guarantee of getting this achievement. After the fight, there is a total of over 40 chests for you
to open.