Using rogue hide/sneak. Saw the spell emote, "You writhe in the grip of agony. You have taken 1211 points of damage." Heh, must have all his harmtouch aa's. :P
While trying to do the slipgear gem quest, I didn't realize that this mob is undead and got nailed. He couldn't see invis to undead, however, and he was yellow to me at level 70. So, ITU helped me get past this dude to continue my "Brave the Ruins of Illsalin" quest. :)