The most important thing is to kill first all cult-members and then the demon. Otherwise it just will reincarnate in one of the cultists and you have to kill it again. I normally summon lots of monstersto keep it busy and have my spellcasters have enough dispeling magic at hand. Also when the fight starts spread your party to avoid that all spellcasters get held or confused. But if I remember right, there was a thread which discribed some more sophisticated battle plans not so long ago.
Another method, which works sometimes is to run directly to the demon, before it casts hold. Sometimes it holds itself then.
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