I really liked the Improved Ilyich modification that Weimer made, for what it does. It made what should be a difficult encounter actually very difficult, and it took me several shots to beat it.
I managed to beat it by luring out the demon and jumping it in the library mainly using magic. I really wasn't that tough. I than rested, summoned some woodland beings and an ogre, hasted, blessed, group strengthed, etc. I ran the ogre in and blocked, and kept the two woodland being back. I than ran in Imoen and began pounded on Zhivago, and used my Kensai against Ilyich and Rasputin. I took Minsc with a magic staff and Jaheira with a magically created club and took on the Jailkeep golem. I used the Woodland beings to cast holds and confusions and managed to freeze up Ilyich and shredded him. Yoshimo backstabbed someone and Minsc engaged. Yoshimo got killed by the Golem and so did Jaheira, but I *barely* managed to finish the engagement.
To pull this off, I had to complete the entire dungeon first, than go back.
Other suggestions would be to just plain improve all of Chateau Irenicus. Suggestions for this would be:
*Buffing up those final theives before your exit to the level of Ilyich.
*Introduce a blocking force by the exit of some signifigance, maybe some high level Shadow Thieves or Vampires.
*The Druegar in the forge room should be much tougher. It's an important room.
*Just in general - get rid of all those wimpy assassins. Only cadre would attempt this mission.
*When the Genie summons that Ogre Mage for the "Noble Path" choice, make it something really difficult!
*The Air Elemental plane should really be guarded by at least one Air Elemental.
*The Cambion-in-a-jar should be really tough, with better armor and sword.
*Ferdanan the Doppleganger is a complete push over. Make him something really, really powerful. Maybe for some extra text, he was the head Doppleganger who helped take over Candlekeep, assisted Sarevok and was "picked up" for info on tracking you. Level 10 fighter / Level 20 mage would be nice
Also - please loose the ambushing Rune Assassins when you sleep (and rename the others to something Shadow Thief related, they aren't Rune Assassins) is annoying more than difficult. More thieves, but make resting easier.
All the best,
Mike