Hi! i just tried this out this morning and this is one of the fastest ways to get from lvl15-20.
what i did was get a party of to most likely cleric and warrior are the best but any class can do just as well. Make sure that you are in a party with a partner that's lvl20 out of the 4 in the party. Then start the abyss make sure that you have rare gear doesn't matter if the other members do or don't. Try to take very little damage just to be sure you do have a cleric.
Also make sure you have a Tinker they make it a little faster with the summoning of there minions.This took me under 6mins first try, also the fourth person can be any class but recommend you use only use warrior, tinker, and cleric to make sure the game moves smoothly.