Any mission done in Co-op on atleast HEROIC difficulty would do, but I'd strongly recommend MISSION 4, because in this mission there are no enemies at all! You just walk around and talk to a couple of people. If you also turn on at least three skulls you'll earn the MY RULES achievement.
NOTE: You will not begin the game with the skulls, you will need to follow collectible guides to locate a couple of skulls throughout the campaign.