Sharing a new method that I found on x360a. Full credit goes to DeadMac.
Original LinkIn split-screen Horde with Friendly Fire mutator, player 1 is killed via friendly fire in a corner, then goes into ghost cam. The 2nd player holds down "X" to constantly swap weapons (players must start with different weapons- Hammerburst and Lancer, for example) while player 1 repeatedly presses in the left stick to spot the weapons. Every 3 spots earns a Reconaissance ribbon worth 125 exp.
Before starting, be sure to give player 2's money to player 1 so player 1 can buy back in when done boosting. Then establish a base and finish the wave. You can quit to menu once the wave 2 countdown starts to earn all the experience from the ribbons. YOU MUST COMPLETE THE WAVE TO GET THE EXP FROM THE RIBBONS!!
This method is much easier with a turbo controller that can turbo the pressing of left stick. Otherwise you'll need to manually press the left stick for each spot. Controller 2's battery can be removed while "X" is pressed, which will continue swapping weapons without pausing the game! (Thanks Diamond Thing!)
I'm letting this setup run right now and I am getting 10 Reconnaissance ribbons every 45 seconds, which should net me 100k experience per hour. Will update solution in the morning with my results.
UPDATE: Setup ran (casual difficulty) for 8h17min and I received just over 1 million EXP!
Important! After about 9 hours and 10 minutes, the round will automatically start and enemies will kill your idlers. Simply restart at the current wave and finish it and you should receive all the experience from the ribbons you earned during the 9+ hours of idling..