Demogorgon fight is something !
Feel free to "spoiler" that if needed : )
At best I managed to get out Saint Kargoth, Archpriest Nulonga, Belcheresk, and Exarch Kazuul.
But Rozvankee is virtually immortal as long as her SI:Divination is on... maybe lightning spells could do something to her (as in chain lightning since she can't be targeted - and I guess she has protection from lightning anyway...) ? Anyway she keeps spawning those horrid Vargouilles (which btw are not supposed to be THAT strong from a monster manual point of view. Even as easy to kill opponents they'd still be an annoyance.). And Demogorgon himself keeps spawning mariliths...
This is the sequence I found to get most enemies out of the way in the least time possible (less than a round, I expect this fight to be extremely short if well done) :
Necro starts alacrity
Fighters stack left of Demogorgon (to avoid a dispel magic effect). Character who remains on the eastern part of the room comes to Necro (necro who is spawned north, said eastern characters moves there to avoid dispel effect too, it was Mazzy here).
When dispel hits, fighters rush on Saint Kargoth
Necro goes 3xRRoR on Saint Kargoth
3xRRoR on Archpriest Nulonga
Breach on Saint Kargoth (breach spell moves a bit faster than RRoR so it needs a few instants so it doesn't hit first)
Spell Trigger 3xLower Resistance on Saint Kargoth.
Cernd moves south and starts alacrity (needed to dispel clouds)
Party pounds Saint Kargoth (someone procs Free Action for all on cat figurine for stun effect from Exarch Kazuul) while Necro unleashes spellbook of acid spell on him. Necro also uses RRoR or breach from time to time to make Archpriest use his SI : Alteration and PFMW
Saint Kargoth dies fast (1/2 round at most).
Depending on where Belcheresk is, party goes either on Archpriest or the demon (belcheresk) after a breach from Necro. Figured it was easier to pound Belcheresk because Archpriest under alacrity can refresh stuff fast, hence our will to make him use his protective spells.
Both fall pretty fast if done correctly (before Necro alacrity ends).
Necro uses wishes as needed during the casting fest, 'cause 1 caster doesn't have enough RRoR nor breaches without wish for rest for this fight. At least, as I played it. Wish also may bring PFME for all, which is necessary given the number of ADHW fired. Prot. from fire at all time mandatory too (it tends to get dispelled often unfortunatly).
When Saint Kargoth, Archpriest Nulonga, Belcheresk, we prayed for Rozvankee not to focus one of our casters. Cernd under alacrity goes Natural Restoral when needed.
IMO, gotta take down Kazuul next, who keeps refreshing his hardiness making him immune to physical damage (ofc), but he's killable, we managed it twice.
Then Rozvankee and Demogorgon... couldn't pass this point, even after 2 or 3 rounds of fight the room is filled with Vargouille x6+, and Demogorgon procs 3 Abyssal Escorts (every 5 rounds unless I'm mistaken). In addition to that, Kazuul tends to go full retard on smite and stun/pushes my characters to the top of the map, making 2 additionnal Abyssal Escorts spot us, and even sometimes locking the character if he gets behind the portal (wayout being occupied by said Abyssal Escorts).
I'm really interested in some advices from those having achieved this insane fight : ) I gave up on this run but I kept a save to replay it sometimes.