I don't use Nenio either, so I lack a full arcane caster, I just have a partial caster in my Eldritch Archer and a limited spellbook in Ember. I do have Nenio in my companion roster, but IIRC you're not allowed to change your party within that dungeon.

...I had no idea Mark of Justice Smite Evil was a thing. I should really go review my party's class features.

I was wondering why Arueshalae wasn't getting Favored Enemy bonuses against her. Either way I don't think my issue was really hitting her since I could stack enough modifiers to do it in theory, it was surviving long enough to take her down.