I'm going Necro since (a) the Set Dungeon conquest is back, and (b) they have the only set dungeons I haven't mastered yet.
Wizard will probably be my secondary so that they can share jewelry.
I'm going to hazard a guess that it is more complicated than you believe it to be, and furthermore that they know more about it than you do.
I believe the fastest is to farm a dense area like Halls of Agony and rack up huge kill streaks on a lower difficulty. Personally though I prefer just running bounties, both for the variety and for the mats.