Hello.
This is what I did. After being forever there,
unshackling minds and killing skeletons, I saw
that I was going nowhere. I loaded the saved game
previous to the entrance to the soultaker's
chamber.
Then, when one of my chars got possesed, I killed
him/her, unless it was my strongest one (melee
fighter), augmented and with Enduring Armor and
Steel Skin from before. I placed her on the door
with a bow equiped (Ebony bow) in case she got
possesed (when she did, she didn't move from there
to attack, since she had ranged weapon, but she
didn't attack neither with her bow, no idea why).
You need to make sure that your strongest char
survives. The rest can die (for now). Once the
soultaker has no one to jump to, he will split
into several shades. You need to make sure to
have your surviving char on the entrance door of
the chamber, so only one (at one point two) shades
can hit you.
Then they will attack you. Kill them one by one.
Assault crystals or potions are very usefull here
since they damage the attacker and heal you over
time.
When there are no shades left (you will probably have to kill the remaining skeletons
too), the cursed Soultaker will appear. Make sure
you are hasted and RUN like hell to the stair
that bring you back to the surface. Go back to
the town (maybe using a Raise Dead scroll could
work and save you time, but I don't know if they
revive with full energy. May be worth a try
though) and once you get your full party again,
rebuff and go to kick Soultaker's ass.
It worked very well for me and was quite fast
(specially letting the casters die).
Note: make sure that your remaining character kills all the shades and you get the message about
the Soultaker appearing (you will see him/her/it)
before leaving the entrance door of the chamber,
otherwise, it will reset the room and you will
have to start over.
I hope it helps, and thanks to those that posted
the order of the orbs. You saved me several hour
of sleep