I think your best strategy is to be the highest level you possibly can before attempting to take on Demogorgon.
This is usually easy enough if you manage to do all of the quests in SOA, but if you're still having too much trouble with him, I suggest trying to advance the TOB storyline further to gain a few more levels.
As for actual strategies in the fight, all I can really suggest without replaying it is to take your strongest fighter (such as
Kelborn,
Minsc or
Sarevok), and cast every single protection spell and drink every useful protective potion that you have, as well as enabling any special abilities, and having them occupy Demogorgon while the rest of your party tries their best to slay him.
Make sure you have spells such as
Remove Parylasis,
Greater Restoration and the like in case your fighter is adversely affected.
In addition, try to hit Demogorgon with spells to remove his magical protections,
Spell Strike is the best example of this, a level Nine Mage spell.
Lower Magic Resistance will work very well here too, after a few hits it should allow any magic users in your party to effectively damage him with thier offensive spells.
Any powerful summoned creatures could also come in handy if your fighter is taking too many hits, and to draw Demogorgon away from any of your other party members.
You could also try looking at the
Strategy Guides for better help than I am able to provide. Good luck.