You can also use soul-ray to take them out in two shots from the stairway just behind
them with a magic stat of about 17.You may not be able to get in two consecutive hits,
but when they charge you they attack from a fair distance away and a simple dodge-roll
backwards will allow you to cast again, or they will try to heal which will open your window up even more.
You can also utilize the Thief's Ring so that there will be no chance of detection, and the ring-of-magical-sharpness helps quite a bit as well.