Longshot; that is a brilliant way of beating him, I had not thought of GoI. So I must admit that you are a much better BGtactician than I. Which I already knew, I am more into the fun and RP, and get my tactical challenges from [url="http://www.europabarbarorum.com/"]EB[/url], [url="http://linetap.com/www/drg/SPCamo-4.htm"]WinSPMBT[/url], [url="http://linetap.com/www/drg/SPCamo-2.htm"]WinSPWWII[/url] and [url="http://www.youtube.com/watch?v=_2GzT1fM9RA"]RL[/url].
Kuroisan, the acid kensai
Spoiler
Kuroisan is a fighter mage who visites you when you have Celestial Fury. Much better fighter then mage i might add, but the most important information here is.. He plays defensive. Attacks only when his defense is up to the task. He has few great items like Robe of Vecna, ring of gaxx and belt of infernal barrier, sadly those are undroppable, and a godly katana which gives him immunity to acid, lowers acid resistance on hit, adds acid damage on hit.. and shots a melf's acid arrow on every target that performs a hostile action towards Kuroisan. Always. He starts the fight with many protections and recasts them while fighting.. This here is important - recasts them! - He focuses on recasting his buffs instead of attacking.
Easiest way to deal with Kuroisan is to breach away his protections (ew. ruby ray spell protections, in case he casts any), unleash physical hell, breach them again and kill him. Protection from magical weapons stops his acid katana fully, acid protection spells or scrolls deal with the backlashes, and Globe of Invulnerability stops them completely (the backlashes are counted as Melf Acid Arrow, lvl 2 spell)
Mage protected with GoI (maybe even a minor one) and pfmw is absolutely immune to everything Kuroisan can do. If you dont want to risk anyones lives, tank him with mage protected with those 2 (and preferably Spell Shield to stop his Breach spell, if you play everything well he wont be able to cast more then one.) and kill him with ranged weapons with characters fully protected from acid. Going into melee with him might cause him to switch targets.
P.S. It -is- possible that Kuroisan focuses on the character with the blade he wants to obtain, i haven't confirmed it yet.