When you boil it down, Mad Moxxi's Underdome Riot is a best-tackled-in-co-op arena challenge that sends wave after wave of enemies out to smash you. Other games call this kind of mode by a few different names - horde, survival, etc. - but they all work the same way; just keep fighting until the enemies run out of warm bodies or you die at their feet. Simple. But what works elsewhere doesn't work here, mostly because the developers struck gold with the RPG elements of the main game and have subsequently betrayed them with this add-on.