Have you deployed all of your Skill Points? With a lv 9 or lv 10 WH you should be able to take it with ease. Even without Ultimate Soulshield for protection. If all potions don't work, keep trying as part of a team. You will get it eventually, but make sure your last attack is single-target and not AoE. I've noticed that sometimes AoE last attacks seem to void quest completion.
Support your friends; share your spares.