Rift Quest:The Wellspring of Power  

Head to Realm of the Fae on expert difficulty. Here fight your way to the second boss, Luggodhan. Take care when dealing with Luggodhan because the Wellspring of Power behind him can aggro during the fight and that will make both the boss fight and the Wellspring event nearly impossible.

With Luggodhan out of the way now you can deal with the Wellspring. Have your group focus its damage on the Wellspring but shift periodically to AoE to keep the wisp population under control. If anyone in the group can dispell the Wellspring's buff it will make the fight much easier. When the Wellspring drops below 20% health use the Obsidian Arrow on it and then finish killing it.