It is a bug. Do you have the second patch for DKS installed? That will prevent the problem, but I think if it has already happened you may have to send your save in to Larian (support@larian.com) to get it fixed (include a description of the problem and maybe a link to this topic).
Lowering the difficulty temporarily should reduce the damage done by Geshniz.
Can you try using Hide In Shadows (or maybe just keep moving; don't recall if there is any cover there), and see if your creature and/or a summon can take her out?
[Not Sure if Bug][Spoilers]Killed Instantly
