My first reaction to this would probably be mob tagging...
However, I recall hearing on Legendary that someone who got to level 90 had been brought down to 87 for that action so I don't know if that's the same thing.
But basically, you got a bunch of people together and the one guy tags mobs, and the rest destroy said mob. You keep doing that over and over so long as you tag first and you get a bit of the xp. You can just grind through all five levels. You gotta find a good area where you can just claim and grind.
That's how the one on Legendary did it and I would assume that's how they did it too.