The way I did these achievements was very simple. Have your main character to be a high level character from level 50 - 69 if you have the dlcs. Using a seperate profile/gamertag train up a character through the story till they can teleport to 'The Underdome' this'll be around level 10 maybe lower. Then go into a split screen game with your low level account as player 1, use your main higher level account as player 2, the lowest the enemies can be is level 12. Play through the small tournaments, then move on to the big ones, this will still take a fair amount of time, but with your high level character things should be a breeze. Good Luck