It's possible to obtain Orontha Ziggurat soul without fighting Garzhad.
If you escape first fire trap in fight mode, leaving upper level
and geting back, the first fire trap will not trigger again.
You can use the same technique(you simply left only one character alone with others in safe location) to steal
the soul and escape before the second fire spreads.
This may be fixed by triggering first fire trap permanenetly.