my favorite strategery is to hold off doing that quest untill i have the cloak of mirroring, then i just put it on my strongest char and let him wazz up to the beholders and they usually kill theirself with their own spells
i belive it refects all deaths spells to, the cloak of mirror i belive refect all offensive spells which is nearly 99% of what beholders do, including the burtual elders, they might take a bit but they will fall to their own spells