I have never foudn the queen herself to be any troubhle, it is the death lord with their spells that wipe me out.As far as sneaking, thief or no thief, you can start combat by the edge of the door, with no one in view, then run in behind a pillar. Combat will end, you can start it again adn run to the spiral stairs, again combat can end. From there you can go up the stairs but it is very difficult to target.
My parties have never lasted until they coudl withstand the death wish, instant death and death cloud spells.
Tools for Wizardry(r) 7
I managed to clear this area with a level 14 Faerie Ninja (with no cane of corpus at that time.)The strategy is to initiate battle in the entrance to the temple then retreat back down the mountain path to just around the first bend.
Typically the 6 lower level scorcerers with follow you together with their band of summoned elementals. Being a path only one or two can get at you at a time. Being around the corner the death lords that are backing up will often not target you. Even if they do, my Ninja was strong magic user and had developed some reasonable resistances at the time anyway.
I cast a level 7 fire elemental in front of me and he basiclly punched out the opposition for me. A grand performance that was a pleasure to watch. All I had to do was sit pack and heal him up.
I was left with just the scorceress queen who had by that time retreated to her temple...much easier.
Although oddly, I then had more trouble with her than I did her entire entire band of defenders. Although, again, another elemental helped me out in the ned.
In theory it should be easier with a full party, but I have not been able to clear the area with less than the same level 14 characters as a full party.