To Unlock this achievement you must find, then beat the Tiny Tina DLC Circle of Slaughter.
This is a side quest given by Moxxie, it is called "Magic Slaughter". She is located in the Flamerock Refuge.
I completed the DLC story before even attempting to beat these quests as the lowest level enemy in the slaughter is 34.
(If someone could comment if they were able to attain the quest earlier that would be awesome)
Once inside Murderlin's Temple, talk to Murderlin and get the quest.
This events first 4 Rounds are 4 waves each. The 5th round is 5 waves, and the 6th round is 6 waves.
There is a considerable amount of elemental damage, especially fire (be prepared). There are also a considerable amount of booby traps that open up throughout the challenge. Personally I failed the challenge twice because I fell into a trap door. (Pay attention to his verbal ques)
The achievement is given after completing the final round "Magic Slaughter: Badass Round"
Here is a list of enemies in case anyone needs this, there may be a few I missed
Enchanted Skeleton Archers
Badass Fire Archer
Fire Mage (Summons: Flaming Phoenix)
Badass Orc Warlord
*NOTE* As IMad Max oo7 has said, completing the "Roll Insight" side quest has fixed the glitch where you cannot turn in the quest with Murderlin