This is the shorter version of all 3 coliseums.
You need to fight 5 rounds with 5 waves of enemies in 3 places.
The waves consist of:
Starter Wave (basic enemies with Skags/Bandits/Psychos)
Gun-Wave: Bandits with Weapons
Badass-Wave: (Not only!) Badass-Variants of Enemies like Alpha Skags or Guardians
Boss-Wave: Any boss
Beware, in some cases enemies are kind of hidden, when you hear no shooting sounds, then they might be stuck anywhere. Just shoot some times to get their attention.
In every case, the enemies won't make any trouble to you, when you load the game with a low-leveled character and run with your main account. I recommend a lv 35 character for this. There will be only 1-2 modifications that makes it a bit annoying, but still it is easy for you to get to the end.Pedle Zelnip
reports: "if your main is an imported character from the 360 version who already had this achievement, then after completing the 3 rounds you'll find your low-leveled char gets the achievement but your main doesn't. Would recommend doing this with a character that's never unlocked the achievement."
See in the comments.