I've been stuck in the last battle in the hell now for a few days. I can get those demons killed easily but Irenicus is quite tough bite for me. Any suggestions/hints how to kill that son of a bitch? I can get him to condition "injured" only. I have two fighters,one mage,one priest and one bard. Boo is also very confused about all this :(

cast protection from evil the instant you can. this will help out with the demons and make you better able to fight Irenicus. Jonny will teleport off to the south, so you can recast your healing/buff spells first. However, before any magic will work on him, you must
a)wait for his non-dispellable? invisibility to wear off
b)breach/ruby ray all his protections
c)cast greater malison.

once you do all that, having everyone in the party (except the mage & cleric) surround him and beat on him should work. have the mage and cleric cast pure damage spells. once you run out of spells, have the cleric join the fight and the mage run and hide. he regenerates really fast, but he should go down fairly fast if you keep beating on him. if that doesn't work for you, keep reloading and resting to memorize different spells.

as a side note, why do you only have 5 members in your party?

One thing you can do is kill the Demons then go sit off out of his sight and just wait for his spells to wear off then send in some monsters to start to weaken him and use you melees behind them. This way he fights them and not you to start off with.

Thank you guys! It helped when I attacked after Jon's spells did wear off. It was glorious victory, no one got killed and Boo was satisfied ;)
I didn't want to take anyone after Imoen was captured/killed. I made well without sixth player and gained more xps. Now back to the Amn, this time with the Darkest Day though.