Geshniz is indeed powerful, she use Magic Blast (I believe) and that's a powerful and especially FAST spell ! So it's important to find a nice strategy plan .
I kill here this way:
Summon my Creature (being a Fighter with Rush Attack & Fatality), summon my Ghost to help heal me and hurl Fireballs .. After that I go to Geshniz and after she starts to attack I fast jump back and behind a corridor/wall, use Healing if needed and use Hide in Shadows (1 or 2 points in the skill only). then I fast run to one of her lackeys and use Charm so he starts to help ME.
After that I run away back and if neede summon my Ghost again and start over the same strategy.
In my case (playing on nightmare diff;) 1 time Charming an enemy makes short work of it.
Geshniz I stripped in parts with Thousand Strikes (just 1 or 2 points in it) ... and my Creature did the final blow via Fatality
