Hmm, now even that doesn't work, it's been patched so the moment you try, the cutscene begins followed by the fighting, no surprise. So, here's how I cheesed it instead...
Now Orin murders the kidnap victim before you can do anything - Halsin in my case, if you don't play it her way. I got a Runepowder cask/bomb and dropped it onto the platform with some assorted smokepowder using Lae'zel's Mage Hand. Gale then drank a Haste potion and cast Globe of Invulnerability on Halsin (and unfortunately it protects Orin as well) then Misty Stepped away ato set off the runepowder with a firebolt. This killed all the Sancturied Bhaalists, leaving just Orin and one of her minions and only one round of Invulnerabilities to be knocked down by an upcast Magic Missile.
