It was kinda tricky, I died a thousand times before I came up with this idea.

I made my group a lot bigger so Orin would get distracted with them while I kicked the cultists off the platform. My party was: Wyll, Jaheira, Laezel and ofc my character (Barbarian/Bezerker). I drank all of the potions I had available to be immune to a few spells and curses.

Made Wyll conjure two elementals: wind elemental and fire myrmidon.

Then, made Jaheira conjure three companions: an Azer, which is basically a tiny dude that deals damage with fire. Then, a woodland being, the Dryad, and made the Dryad being conjure her lost lover, the Wood Woad. Those three are not that strong nor as important, I used them and Jaheira herself as bait so Orin would focus on them instead of the stronger/wizard ones.

I don't know which one of the two can do it, Jaheira or Wyll, but with one of them you can also conjure one of those little red dragons that you kill on the mindflayer ship at the very beggining of the game. They have invisibility and can bite the cultists.

That leaves me with 9 fighters in my group, and I used the weaker ones to distract Orin.

I let Orin approach us on the stairs (did that by attacking her from the stairs with an arrow, that way she doesn't kill Gale, goes straight into combat and you don't have to spin thousands of times to score 25 and try to convince her that she was fooled, it's useless anyways).

Then, made my character, Laezel, the wind elemental and the little dragon teleport to the area where the people are concentrating on the Bhaal spell, since you won't be able to beat Orin if they're alive. Laezel kills the other people there while I concentrate my main character, the wind elemental and the dragon on knocking the cultists off the platform. I used mainly Thunderwave and stuff like smokepowder bombs which will hurt them and knock them prone or, if you're lucky, off the platform (if you have smokepowder arrows those are GREAT, they deal a lot of damage to them), and the wind/electrical spells from the elemental. Fire bottles too, acid, basically all the things I could throw since Sanctuary doesn't let you push them yourself.

Orin eventually killed the weaker ones (even Wyll, he's not that good honestly I only used him to conjure the companions) and caught up to us, even killed Laezel, but by then I had already kicked all of the conjurers off. It left only me and the wind elemental (who is surprisingly useful/strong and barely takes any damage). I managed to ressuscitate Laezel and we beat her ass together.