Best way to kill her, is before the big fight.
Either before you finish the goblin matter (if the goblins are not dead by ACT1), or before Zrell moves to her room afterward.
You can sneak into her room, killing the 2 zealot at the balcony outside her room, then bait the troll guarding her room with illusion and kill it.
Don't forget to deal with the scrying eye afterward.
Then proceed talking with Zrell, until she gave key to balthazar room.
Now she moves to her room, with no one guarding her.
Cast silence on her area or hold person, then kill her with melee attacks asap.
OR
Bait her with illusion to Balthazar room, and kill her there.
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I'm not sure if by doing this I'm missing some reward or treasure.
But at least, that's how I killed her.
