I was messing around with the quest "Take Down Tethyr!", and I found out how to keep him in the bay until killed.
First get the quest from Sergeant Amelyn near the fountain in Theramore Isle, and complete it up to the part where you summon Tethyr.
Accept the quest, then abandon it and run outside of the island, stay there for about 2 minutes.
Run back and it should say there are marksmen remaining, but there aren't, so he patrols the docks looking for them. He should stay until he is killed.
Not sure how this could be useful, but its fun seeing all the level 30's trying to kill him.