Only remembered to send the owl after saving the paladins. Quest doesn't close for some reason, even after receiving the rewards.

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

First off, you have to sneak up on the shriekers, took me a lot of trail and error to figure that out.
I went ahead and killed the silent monks to get them out the way before engaging with the Magisters.

Regarding engaging with the Magisters and saving the Black Ring Prisoners I tried almost all the suggestions below and they didn't work:

I tried the oil and the bless on prisoners didn't work
I tried teleporting them to the side room, closing the door and blocking it with objects, he still was able to shackle them through the door.
They were always dying I couldn't heal them enough or kill Vorrh fast enough.

What worked for me was actually teleporting him far way from the the prisoners and then having one man in my four man party hang back and he was taken out the battle. I then proceeded to engage trying to kill Vorrh (of course after I killed the other Magister) and I had the one party member who stayed out of the fight healing the prisoners constantly, so I was switching back and forth between healing and each attack on Vorrh with the rest of my party. Took forever but it worked.

If you're having trouble, you can close the back and front door and kill the silents alone. Put something to block the way of the back and Reimond can't run away from you