My party consisted of a halfling wizard/elemental savant/archmage focused on evocation spells, a human samurai/kensai, a human barbarian/fighter/rogue with the smoking eye, a human rogue, an elf druid, a pixie sorcerer/incantatrix, a half-orc cleric of Kord, and Nidrama. The group was at nearly full strength; they breezed through the majority of the prison and did not make the mistake of trying to open the cells. They still had a good amount of healing spells left available to them (on a side note, a sorcerer with disintegrate is nasty). All of the PCs were at or around 18th level. The entire party had been mind blanked from the marilith's stone and had energy immunity or protection from energy against fire, the bbn/ftr/rog had been polymorphed into a troll, and the druid had the majority of an earth elemental swarm. The cleric had several buffs active on him (righteous might and spell immunity [blasphemy, horrid wilting, and one other] among others that I can't remember). All of the front-line fighters were under the effects of displacement as well.
When I ran it, I decided to roll randomly to see which form Adi would take each round. He spent most of the first part of the fight in his angelic form. He opened with blasphemy, which paralyzed the rogue, the samurai, and Nidrama (the cleric was immune and the smoking eye allowed the bbn/ftr/rog to be unaffected). The druid cast remove paralysis on Nidrama, who in turn did the same to the samurai. The samurai went in with his swords (+5 keen speed weapons IIRC) and did about 100 damage in one round, but Adimarchus switched to his demonic form and destroyed the samurai in one round.
Though the battle was long and grueling (perhaps 20 rounds or so), the rest of the fight went heavily in the PCs favor. Two dual-weilding rogues with sneak attacks are nasty, especially when one is polymorphed into a troll, is raging, and gets a bonus to attacks against Adimarchus with weapons that can penetrate his DR. Not to mention the cleric was wielding a blessed holy greatsword that was bane against evil outsiders. The druid kept summoning more creatures to use 'aid another' actions on the party, increasing their AC and attack rolls by an average of +8 each round (lots of huge and large elementals that could reach over the PCS). Adimarchus could hardly hit the party members, and his word of chaos was ineffective because all of the front-line fighters are chaotic. Throw in the archmage hurling powerful evocations being maximized by the incantatrix (readying an action each turn to boost the archmage's spells...his Spellcraft rating was through the roof) and the angelic form quickly fell. And the demonic form, which was actually subject to criticals and sneak attacks, fell even quicker. The PC's made all their saves against the imploding strike.
So in the end, the group only suffered one casualty. Needless to say I was surprised.
