I did something similar to SixOfSpades tactic, scouting with tanks. I had Minsc (Spider Bane) and Kivan (two Bastard swords+1 Ring of Free action) gulp down a potion of cloud giant strenghth and an oil of speed each. Meanwhile I cast Remove Fear and Bless on the party (while they were still there).
Now I sent them both down. The priestesss didn't last longer than two seconds and after that I started killing the robe-guys. One of my tanks was always chased by the demon and only landed a few hits while the other was free to hack away those evil monks. Since his ranged attacks are not that bad the demon did little damage to Minsc and Kivan and when I had finished the last robe guy I got down with the rest of the party and hacked/spelled him away.
My tip: Since this is one of the last really tough fights be not afraid to use up those tons of strenghth potions, oil of speed, potions of heroism, potions of invulnerability you have aquired during TotSC. You won't need them when the game is over.