Yea, I just used the one where you have to kill 50 elites. Kill around 47-49 (don't get the challenge) then save and quit your game. Go to the sword base level and kill everything with the target locater and it will give you the challenge. Then simply disconnect and re-connect from xbox live and it will reset the challenge (to where you had killed 47-49 elites) and you can keep getting the challenge cR, and getting the challenge over and over again. Then just re-load the checkpoint and repeat. You'll get around 1,500 cR each reload of the checkpoint until you reach the cap of course lol.