-Start with a level 1 character.
-Give him a BFG9000 weapon + gem of ease.
-Start a brand new adventure mode game with that level 1 char in T6.
-Have 3 other level 70 chars join so your exp bonus is increased by an additional 33%. The game level should stay at level 1 (making it easier for the noob char to survive).
-Start clearing bounties. The noob char will have to stay nearby, so he should concentrate on assigning "get me out of here" skills as he gets them. He should also don gear as it becomes available.
-Just after level 20, switch out your helmet for one with a socket with a giant ruby in it. Bonus points if it happens to include the
Cain's Fate set, preferably with a RoRG so you only have to wear 2 pieces to get an additional 30% exp.
-Once you hit level 45-ish, add two pieces of the Cain's Destiny set (with "level requirement reduced by 25" on each piece) to gain another +50% bonus experience.
-Reduce difficulty one notch when the killing slows down too much (or the deaths come too frequently), repeat as needed.
This is the fastest way I can think of to level that doesn't require micromanaging a bunch of "+xxx experience per kill" items.
--Patrick