The easiest cheese for this fight is to have a character with charmeleon cloak and teleport and a character with ranged damage.
Move your ranged character to stonegarden and walk to the edge of the cliff near the eternal fire. There you can shoot down without taking aggro.
Now kill the Wilent Watcher standing outside. Now take you cloak character, walk through the house while invisible and move outside where ther ship is.
Then sneak (so you don't get spotted) move to the door and teleport the character you want to kill outside, and WHILE teleporting quicktravel away, so you don't start combat.
Now you can shoot Vorrh/Reimond and they don't even realize they are getting shot