I just did this fight (tactician level), without harpers, because they were killed earlier when Isobela was abducted. In a a first try, Jaheira was too anxious and got caught in the crossfire as the party advanced into the tower. For a second attempt, we changed tactics. First we covered the stairs with grease from grease bottles and threw some smoke powder satchels in the grease. This is possible without triggering the fight. The fight can be triggered with a nice AoE spell. We picked confusion. We then stayed at the top of the staircase, near the gate, to pelt the enemy from there. Some of them will come rushing up the stairs. Pick a good moment to light up the grease. We were attacked with Arms of Hadar spells. We retreated even further, to outside the gates. This made the enemy move through their own Arms of Hadar area. Even the enemies posted on the overhead beams climbed down to rush towards their demise. We kept bombarding the enemy from a distance until they all died. Disciple Z'rell died somewhere on the stairs from all the environmental damage. We kept Jaheira away from the fight, she could not contribute mucht, aside from an entangle spell at 18 meters.
So after trying out a different approach the fight went much smoother. That can probably be said about almost all difficult encounters in the game, which is nice.
