Is highly underrated, but a level 4 conjuration spell called Guardian of Faith is also a great one, Shadowheart should have it in her arsenal.
If you cast it between your group and Orin's group, and then hide behind it past the staircase, it will detect and kill all the invisible minions that approach you.
If you are not a dark urge and want to deal with Orin easy, you should prepare either by scrolls or high level spells such as Magic Missile and Curriculum of Strategy: Artistry of War between two casters to drain her unstoppable, then whoever is next on turn to fight Orin can just unleash everything they got, Orin is really not too hard once you grasp this concept, if it wasn't for unstoppable she would not even feel like a boss fight.
WIth unstoppable having more counts on higher difficulty such as Tactical/Honour mode just means you plan ahead by having level 6 magic missile infused with lightning sparks and Curriculum of Strategy: Artistry of War scroll in your possession to help drain all 12 charges of Unstoppable and still be able to land some damage on her, you are looking at making up to 18 hits or there abouts.
I hope this will help you in your future fights somewhat.