(You need to be able to fly!)
There is a quest in Sholazar Basin called that gives you an item and tells you to go to a place where mobs have VERY fast respawn time. (Coords 49,36)
The quest item "calls upon the wrath" and instantly kills the mobs for about 10 seconds. All you have to do is tag them (I did this with rank 10 Arcane Explosion), and they will give you XP before they are instantly killed by the quest item. (500xp aprox. per mob non-rested).
With 2 other party member I was able to do this constant even though the quest item has a 3 minute cooldown. As well as others coming to do the quest and using THEIR item.
They also give LOOT, cloth + silver, you will have too many mobs to know what to do with.
Screenshot: