I was actually drinking a little bit of beer, on my free night a few night back whilst playing through this and I have to admit that I don't recall these battles in that much of a detail as I didn't make any notes.

#1. Summon-tactic:

Whack the entourage, summon, summon, summon. Smilodons are great, ditto for Clan Spirit. Arcane summons in trolls & Yuan-Ti's work great as well. Drop regenerations & PfF's for bigger summons (we always buff the CS). Use a tank (I used the DD ofc.) to whack it every now and then. When all PFMW's are exhausted, re-buff your crew, enter & anhilate the dragon. Without protections it doesn't last for long. The problem with this tactic is that it looks like the dragon has an (unending?) pool of "lower fire resistances" so after some time the dragon will lower your tank's fire protections & dispel, then you have to re-up with PfF, Res. F&C, PftE, PfE + rings and even that won't get you to proper fire resistance level so in the end your tank will be very, very vulnerable to improved fireballs. Then again, once it's out of PFMW's with a competent frontline one can easily afford not to aggro with the char that took majority of the lower fire resistances.

#2. Trigger-tactic:.

Whack the entourage. Summon for distractions. Use triggers for instant breaches when the dragon is busy casting something else, so walk in - release trigger - walk out of sight. Effective ways of doing this include at least casting a mirror image (or imp. invisibility), so Dragon will cast true sight, quaffing a potion of magic protection (or cast magic resistance with Cernd for example), so it will cast LR or tanking with summons and using a missile weapon so the dragon will fire storm to the character using the missile weapon (we used dwarven thrower). Once PFMW's are out, buff up fire resistances, enter & obliterate.

#3. Power-tactic:

Whack the entourage. Dispel spell protections. When the Dragon starts taking damage from spells (most likely after entourage is done). distract, enter dragon's sight with aura clensing's & IA's and drop as much damage via spells to the Dragon as possible. So CO's, AA's, Flame Arrows (lightning bolt is probably better, COC's and chain lightnings (should work)). And should you have Cernd also implosions & nature's fists should work. Two competent casters with maxed casting time should do the trick, especially if it's supported by triggers, sequencers & Chain Contigencies and competent frontline whacking away under IH & CS (as the dragon will need to renew protections).

#4. Dispel-tactic:

Relies on specific characters in the party / high enough levels. There's a few possibilities for dispelling (/breaching) the Dragon without Necro. A high-level enough Cernd can (read: should) be able to dispel the Dragon and he's unsilenceable. This also works with Jaheira, if you have her "perform ritual" active. The other options include dispelling with inquisitor with it's innate ability. And if you have a thief (pure class / dual class with Use Any Item), you can cast "breach" from a scroll. Last but least one could use a cleric here, as they have faster level progression. And you can cast dispel from off-screen, so without visibility. The Dragon moving all the time makes this difficult though.

Max summons, so 2xGreater Yuan-Ti, Greater Djinn, CS + Sunnis. Greater Yuan-Ti goes for Belcheresk, the other goes for Kazuyl, ditto for Sunnis, Greater Djinn takes Rozvankee and CS goes for St Kargoth (under IH, PFME, Skins + regen). The rest are just hasted. The micromanaging required here to make everything work makes us insane.

Max prebuffs. Including barbarian essences for both Kensai + DD. CC 3xRRoR "on hit" for F/M. She also takes 5 barbarian essences, shield of harmony (we switch with supreme shelter every now and then), Peridan, boots of the north, ring of gaxx and improved dragon helm. Protection from level drain scroll to, well, protect from level drain. Start, launch trigger with llarloch's minor drain to debuff Nulonga & then F/M moves to whack demo, once she gets aggro we move her to near St Kargoth. Once St KArgoth is whack-able she whacks and then she tanks Demo + Rozvankee through the fight, all the way to the end. And at one point she tanks 5-6 Vargouilles + a bunch of Mariliths. She also has potions of cold resistance (2-3) and potions of fire resistance (5-8) we keep quaffing potions of regeneration in the beginning, she must've taken perhaps 10 or so. She gets stunned once & once she gets held. Doesn't matter, she's quite difficult to hurt with everything that she has. Plus we take 3 or so RvE:Regen's throughout the fight. While stunned/held she only takes fire damage.

Imoen is the class in the beginning, she goes IA, Breaches Nulonga (Who is then whacked into oblivion by IH'd Kensai & Jaheira under CS, though we have to use 2-4 breaches here), then she debuffs Belcheresk, breaches, lowers it's resistances (ST 3xLR) and then drops 12 Magic missiles on it with about 15 acid arrows (10 + both sequencers). Cernd aura clenses and then drops 2-3 unleash energy's on Belcheresk, which then falls. Then we 3xRRoR St Kargoth,3xLR + Breach and drop 30 flame arrows on it + our remaining acid arrows. Jaheira, under SoA + PfME goes to give it a whack. Unfortunately CS falls here, it was on St Kargoth, otherwise we could have natural restored it to full health.

Next up is Kazyul, who is battling Sunnis & Greater Yuan Ti (greater elementals don't get knocked back, that's why they are really good here - for the fight is much more manageable that way). Imoen breaches and Cernd summons another greater elemental to help our summons. DD whacks the spawning Vargouille (just one, under CS) and then moves to whack Kazuul. Then rest of the frontline moves in, it activates "hardiness" and we can't hurt it. We're out of breaches. F/M is tanking demo + rozvankee so she's no help.

At this point Imoen is out of 1st, 2nd, 3rd, 5th and almost out of 7th level spells so we start wishing. We get Wish:Rest maybe after 8 tries or so we follow-up by re-casting IA and then Breach (x2 perhaps) Kazuyl, which falls quite quickly once it gets targetted with 26 APR with natural 20 rolls. De-breching with -5 and coordinating Criticla Strikes is deadly. Imoen follows up by starting to summon.

Then it's just Rozvankee + Demo.

F/M moves near the staircase to try and feed some spawns to the rest of the crew, since they can't attack her as they can't all fit, but they just stick around her. Demo launches lower resistances + fire storm so we are in a bit of a pickle for some time. But we are prepared, as in we have active regen + batalista's passport + supreme shelter to temporary up the fire resistance for further 75% Plus the potions. She gets stuck there, with Rozvankee & Demo below (+ Mariliths), Vargouilles on top. Her HP's drop quit fast so Cernd moves in and heals with natural restoral. It's enough. Fighters move in from the top, clear out the Vargouilles (one of DD, Kensai or Jaheira is always with CS (three capable fighters with one using CS the Vargouilles don't last for long) and once cleared move to the center we go where we whack the Mariliths, Imoen mostly breaches them (from scrolls).

We dodge fire storms a little bit and a few "lower fire resistances", whack a few more waves of Vargouilles and Mariliths. We give our best shot to separating Rozvankee & Demo here (using summons to tackle Demo) but it doesn't work, however Demo is quite conveniently single mindedly aggroing F/M, even after it's distracted by a summon. Rozvankee runs out of SI:Divination. We true sight & whack it immediately (it doesn't launch a trigger, though our frontline had PFME's active, probably from the Wishes in the beginning of the fight). Then it's just Demo. Debuff (not sure if it works but we do it anyway) and towards the end Imoen doesn't have anything worthwhile to cast and frontline is almost out of Critical Strikes so she Wishes and we get our rest #2. Yay!!! At this point the fight is won, we are only down a lot on fire resistances but with our capabilities the renew spells will easily carry us until the end. We re-up fire resistances as much as possible (Cernd aura clenses and runs around casting protection from fire & resist fire & cold's) and front line whacks demo with CS. Cernd drop CC's for whole frontline together with resist fear (from Imoen) so F/M switches to Githzerai Blade (she has 5 Critical Strikes). Demo against 30 APR with natural 20 rolls doesn't last for long, still it's perhaps 3 rounds or so. It's quite tanky. It seems like it takes forever. In the end we give Noble Staff of Fire to Jaheira, so in a pinch she can equip it to dodge fire damage (she's at -10% or so without the staff). Doesn't matter. We hear the sweet sound of Demogorgon's wail while our group of mostly mere mortals thusfar whacks it into another plane. Then we finish the remaining mariliths. F/M, who's been taking the damage takes giant strength from Jaheira & belm off-hand and goes on a Marilith whacking spree.

We get our superpowers. We check the difficulty setting 5 times. Yes. It's on "insane". This took about 4 hours of constant micromanagement.

Our whole crew including PC gets Blessings(?!?!?). On my other runs PC just gets the ring. Well, we don't complain.