Watchers Keep last boss (Imprisoned One, Demogorgon) trouble ?

[Regulas]

SPOILER ALERT
After trying and dying about 6 times I figured out how to kill him fairly easy. When you go through the portal he will talk to you, tell him you plan on reading the ritual scroll. he will back down and give you time to prepare for the fight.
Turn off your AI to stop them from changing targets or casting unnecessary spells
I spread my 6 characters around the in a circle and buffed. All 3 of my mages have simulacrum then i cast other buffs.
Summon the 4 most powerful creatures you can, Planetar, Jaheira summoned a greater elemental and so on into the center.
My fighter has the helmet that can cast simulacrum once a day, i cast it then read the ritual scroll. You are now trapped in the tower along with the demon forever unless you side with I think. Anyway start the fight.
As soon as he materializes pause and have everyone attack only Demogorgon. Ignore the summoned demons. Mazzy and me along with the 4 summoned monsters from my mages went melee on him while I directed my mages cast many spell removals at him to debuff him and as many high powered spells like comet at him as I could.
Demogorgon went down pretty fast.
If the fight lasts too long Demogorgon will call upon his minions and your doomed or at least i was every time.
Hope this helped.

[JODEGAFUN]

He dies fast, and i found this fight is not the hardest in TOB. Key is to focus all on him and not on his minions.