King Chemistry's solution is good but there is faster : choose the mission Uprising, set the difficulty to Heroic, plug in a second controller or invite a friend and let's roll !
The player with the overshield simply need to speedrun the mission, using his camo when he comes by a pack of ennemies to simply skip them.
Here's a video to show how to speedrun the mission in Heroic without anything else than melee (obviously, it's more comfortable with the overshield).
ps : It took me less than 3 minutes using this method.
EDIT : Gameclip got deleted from my XBL, and I can't upload a new one for now as I don't have 2 controllers anymore...