That's a difficult task indeed.
I would first try setting up a chain contingency to fire out all my most powerful summoning spells - let the summoned monsters deal with the marileths and other demons Demogorgon conjures, and focus all your energy on Demogorgon.
I would set up some spell triggers and spell sequencers to:
1. Eradicate his spell defenses...pierce shields, warding whips, ruby rays, pierce magics and breaches.
2. Lower his resistance with three consecutive lower resistances.
3. Then some more conjured creatures to distract Demogorgon (maybe a Mordenkainen's sword and a few nishruus or something,) while you unleash your most powerful magics on him. Dragon's breaths, comets, abi dazims (it shouldn't affect you too much if you cast prot. from magical energy on yourself beforehand,) and other mass-damaging spells.
Sounds like you have quite a fight on your hands. I'd be interested to know how it goes so keep us informed.
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trux
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