Credits for this solution go to Scotty from Xbox Achievements.
You will unlock this for completing all three large challenges in all three coliseums. To do this with ease, though the tedium can't be avoided, you should do it in splitscreen with a second low level account.
Load the game up on an alternate profile (a Silver account will suffice) with a new character and complete the tutorial with that character so that you can gain access to the fast travel New-U station in Fyrestone. Once you are at this point, quit the game and then start it up again with the same character, only this time sign in your main account as a second player on a second controller and have them join the game, making sure the alternate, low level character is the host.
When you then play the game, the enemies will be at around level 10 so if you use your main character of level 50+ then the enemies will die in an instant and their damage will pose absolutely no threat to you whatsoever. Then you can play all of the challenges like this for an extremely easy time.
Of course be aware that if you die you will have to start again from the beginning of a challenge and they each will take around two to three hours so don't become too complacent as having to start any again will be very disheartening.