So i was questing about 20 mins ago in the Dread Wastes and noticed that the Not Fit to Swarm quest gave me a companion called the Angel of Death that has pretty much a god come down, hit everything for about 150k Dmg, and then put a DOT on them that ticks for about 20-35k roughly.
Although it has a cooldown of 1 minute, the math it generates could potentially be 10-15mil XP/Hour, depending on kiting mechanics, or healing/tanking through it.
So my theroy is, if you you gather up multiple groups of the smaller mobs (or bigs ones as well) - each pack is about 8 mobs with 160k-ish health - and gather 3 packs (feel free to go higher) together and have the angel come down and kill each mob instantly for about 7k xp each unrested, you can profit from it.
Generally i get about 180-200k exp from each session that takes about 2 seconds to kill. And multiply it by 60 (1 minute cooldown) comes out to roughly 12mil XP/hour.
Easy
Edit:
The zone where the mobs are are in The Clutches of Shek'zeer