Remember their Spirit Barrier buff that will make them absorb most damage if you don't wait for it to drop.
Return to the Tower of Umber and collect 5 pieces of shattered core. The first time you click these, a mob (or minor encounter) will spawn and attack. After the mob is dead click the gems a second time to get the update. Known locations:
-926, 1, 472
-947, 6, 482
-931, 7, 505
-909, 4, 508
-944, 6, 525 (tucked a bit under the rock)
Repair the core of the Tower of Umber by clicking the large orb at -961, 14, 495 .
YOU WILL HAVE A 2 MINUTE TIMER AFTER THIS STEP!
You may want to clear the immediate area of mobs before doing this step.
Encounters of mobs keep spawning and attacking the core. Get aggro quickly and kill them before they kill the core!
If the core "dies" then you will have to restart at the "repair the core" step again.
Speak to Zeeil at the Tower of Umber to complete the quest.