In the hub of the Underdome DLC, once you start it go to the billboard. It will have one quest called "proof yourself". You must finish all 3 of the smaller maps (Hellburbia, Angelic Ruins and the Gully) before a curtain opens with 4, 5 and 6. (Which are the same maps but harder)
No matter with how many people you will do this, this DLC will take a LONG time to finish. Each bigger map will take anywhere from 90 minutes to 2 hours to beat. Per map!
Here are the facts:
*Do this with friends. Alone this will be as dumb as fighting Terramorphous with level 1 gear at OP8 or Crawmerax with starter weapons. It can be done solo (kind of!) But more on that later.
*Bring good gear with you. Getting killed sentences you to a penalty box (in which you can spectate for the rest of the round. If you and all your team mates are killed the entire round will be reset!)
*Like the first 3 smaller maps, each round will have a certain enemy type.
*In the first set of maps, at one point you had one or two modifiers to spice up the game a bit. In these maps, more rounds have these and there can be up to four to make it all a bit harder. These range from certain types of guns doing more damage, enemies being faster or you having no shields or health gets drained slowly unless you kill enemies or there is moon gravity so you and enemies jump a bit higher.
*BUT! There is a way of making this all a bit easier. The game levels with you. What I mean by that is that when you play the arena at level 8, all enemies will be around that level. And this is where you could make this achievement easier for you and your buddies. And yes, with this method you can also play the game solo!
*Have at least one buddy on a very low level. You need to have to be at least level 2 to start the arena quest. (Thanks REDWINGS05 for pointing that out)
But enemies will be at least level 12!
(Log in with the profile you will have the lowest level, plug in another controller with the higher level and log in)
*Finish the whopping 20 rounds one each of the three maps(4, 5 and 6!!!) , and to be sure: With that one player on a low level! It does not need to be done in one sitting of 6 straight hours, but just to be sure with the same character hosting the game.
*After the final wave of the last map you need the achievement for finishes, the achievement pops for all players in the game.
Good luck and patience to all trying this one. If you give thumbs down, tell me why so I can edit or add information/tips.