I assume this will work with the ending of any mission, but I did it on Bitanti Village as I was going for My Rifle Is My Best Friend, for an extra 1000xp each time for completing the mission only using a rifle.
Simply make a save right before running through the door at the end of the mission, run through, and reload by pausing on the ending cutscene. You'll get 4,000xp every 7 seconds, or ~34K every minute. If you check the timestamps on the saves in the video, I started at level 30 and hit 50 34 minutes later.