For this cheese, you will need;
One character with Sleight of Hand
One scroll of Dimension Door
One Mage Hand spell/scroll/Gith ability
One Greater Invisibility spell/scroll
One of the runepowder bombs from the Ironhand Gnomes
A number of Haste potions, depending on your party

Drop the bomb on the steps and have the Hand move it across to the far side of the platform. Be careful that it isn't blocked from line of sight by the altar. Cast Greater Invisibility on your Rogue or whoever has Sleight of Hand and have them hide and sneak down to Orin. Now have them pickpocket Orin and steal her key: an ordinary Invisibility would drop at this point and start the cutscene and fight, which is why Greater Invis is needed here. Whoever needs Haste potions because they don't have Alert or something else to give a large enough bonus to Initiative should drink them now. Click on the altar to free the prisoner with Orin's stolen key. This will trigger the cutscene and the fight. Have the sneak immediately Dimension Door away with the prisoner, out of the radius of the bomb (and it's a lot) and the remaining characters use bows, crossobws, cantrips, upcast magic missiles, anything that will knock down Orin's Unstoppables (and if you have any spare, that of the nearby Bhaal cultist too) Now use a fire arrow, firebolt or similar to set off the bomb. BOOM! All the sanctuaried cultists are dead and Orin has lost a lot of her HP and her super power of needing to be hit eight times. It's possible that the shooter gets partially caught in the blast too, but it's worth it.