SPOILERS BELOW!
This achievement requires you to beat Round Five of any Circle of Slaughter. The one I got it one was Bandit Slaughter Dome located in Fink's Slaughter Dome, which is in The Fridge. To unlock this, you must progress far enough into the story to the point where Sanctuary is now flying in the Highlands and you can fast travel there again (Not sure if you have to be this far, but that's when I went. To get to Fink's Slaughter Dome, fast-travel to The Fridge and it's literally right in front of you. Now, the Circle of Slaughter is basically survival (Moxxi's Underdome in a nutshell.) You will receive a quest to start on Round 1 when you enter. You have to start on Round 1 and proceed with the other rounds until you reach 5. As you go through each round, enemies will get tougher, Badass enemies and other unique bandit type enemies. As you progress through rounds, they will add more waves, specifically starting at Round 4, so Round 5 will pit you against 5 waves of enemies. Make sure your stocked up before you enter and start the round because you cannot leave the arena once you start. You do not have to do them all at once, as once you finish a round, your done with it forever. Highly recommended in co-op for reviving and a little exploit if needed. Once a person dies, they spawn back in the lobby of the arena and cannot re-enter. If, playing in co-op, the game goes on until everyone dies. To the left of when you spawn, there is an elevator that leads to a spectating area for everyone who died. You can shoot through the metal bars and still help your partner(s). It's far from the fight, so use a high powered sniper. Some action skills, like Phaselock, will still work while up there. Hope this helps!
EDIT: Thanks to RedemtionOWNAGE for confirming this, the enemies remain in the Level 20-24 range, so you could wait until your in the 30's and then attempt this.
EDIT2: The glitched waves are for people dying. For some reason if you fail and then instantly retry the quest it doesn't recognize that the rounds are supposed to keep going. All you have to do is save and quit, load the game back up and then everything works fine. Thanks to FreakOfTheAbyss for finding this out as a lot of people are having problems with it.
EDIT3: You must complete all 5 rounds of the slaughter dome in order to get the achievement. You cannot simply jump into someone's game who happens to be on round 5. Rounds 1-5 must be completed on a single character in order for it to unlock. Thanks osubluejacket for finding this out!