I found Seonghas at 36.2, 19.2 and when I first attacked him I used my bow. He died quick, but the quest did not advance. He respawned real quick and then I equiped the staff and I hit him for 1 damage and he died. He was Orange to me, but I think if you use the experimental weapon, he will always die with one hit.
Follow up: Did this with my L36 Enchanter and Seonghas conned Yellow to him. My pet hit him twice before I made him passive and he did damage both times, but Seonghas died after I hit him with the staff for only 1 damage.
Edited, Oct 13th 2007 9:52am by Friml