I can't seem to overcome those demons! My party consists of:
Imoen
Aerie
Minsc
Valgar
Mazzy
and me 16th level Stalker

Any words of advice before I get killed again (5th time!)?

Well, as this is the last battle in the game (before TOB, anyway) I'd suggest that you use absolutely everything ya got! Haste, Protection from Evil 15' radius, all of your Strenght enhancing potions, Resist Fear, Bless, Chant, etc. (Too bad you have no Paladin, as Caromyr would be very effective in this battle...). You can use spells such as Khelben's warding whip, Ruby Ray of Reversal etc on Irenicus to remove his spell protections. Melfs Minute Meteors will hit anything iirc, so Aerie and Imoen can throw those.

You've got 4 fighters, so this battle shouldn't be that tough... What sort of weapons do you have? By this time, you should have all the best equipped such as Crom Faeyr, Celestial Fury, Daystar, Staff of Magi, Silver Sword (Vorpal blade), Equalizer etc. With these weapons, and the buff spells you should cast before the battle, you can't lose

Move out so you don't have to fight them all. And put a few monsters in front to keep em all from following you. I found I could sequence it to fight 2 of them, stand on some stairs and heal, go back for two more and if you are lucky Irenicus won't find you while that is going on.

Just a followup to this topic... I have a couple of questions of my own:
(There are probably a couple of spoilers cointained in the questions)
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1: I found that the best way to kill Irenicus in his regenning stalker form is to turn into the stalker, in order to get the higher rate of attacks. However, my stalker keeps on spontaneously dying on me, just when Irenicus-stalker is about to die itself! I decided to kill Sarevok, rather than turn stalker for him, so I wondered whether this would have made my stalker-transformation unusable...

2: Whenever I kill Irenicus, I just run away from the demons (and his reflected forms), and leave them to feast on a bunch of summoned creatures. I dont bother with them after that. Do I have to kill them in order to kill Irenicus?

3: This isnt actually a question. I just wanted to say what a blQQdy good fight the end battle with Irenicus is. I havnt been challenged like this anywhere else in the game. Awesome fun! Cudos to the programmers.

You don't have to fight the demons, if Irenicus falls it's sufficient!
The easiest way to beat the evil wizard is to summon up some pets to keep the demons busy and flee south down to a staircase. Irenicus will be teleporting on the platform counter-clockwise. Now wait for his spell protections to wear off and conjure up some small creatures now and then to send after him, as will use up his high-level spells on them. After a good deal of time (and after you've run out of summoning spells and items) use every defensive spell/potion you have on your party and CHARGE! When you have brought down the Slayer to Badly Injured let your strongest (referring to melee) mage use any time stop spell (remember that Limited wish also allows time stop once) he has access to and whack Irenicus (Imoen with CF is very effective). This should make it easy to defeat the Exile even with a low level party.

The hardest part of this fight is the demons. Irenicus is easy. I used my fighter, Korgan, Viconia and Hear'dalis in close combat. Once they were dead it was a breeze. Easy.

Hmmm i have always thought that this fight is easy..even the first time i got it on the first try..no cheat..and i didnt even have any of the cool game stuff like Carsomyr or the like. My mage(lv 17) just kicked some real demon a*s in the start..and then i had a fighter with boots of speed running around in cirkles with Irenicus after while i fired every arrow and spell i had...And then i had Cernd Greater werewolf move in for the kill...easy fight..

Oh and the Slayer/Stalker dying thing..If u stay in that form to long the Essence or Blood of Bhaal overwhelms u r something..u die anyway..u take some damage first and then u die.

I killed him at the weekend for first time. solo f/m/t.

I seemed to have unlimited time stops so that it was no problem. As soon as I finished casting one, I cast another. Meleed the demons and then removed all the protective spells (true sight, breach) and he was dead.

Traps did not work for me.

the priest spell aerial servant works wonders against anything. 1 of them took out bhodi on its own and two of them took out a pit fiend in one strike. they are some of the best monsters. use these to tye up irenicus or his demons and then have ure mages cast oblivion

don´t bother with the demons, you just need to kill little jonnie... easiest to do with half a dozen skull traps (fired by contingency, sequencer and trigger - he should be dead before he has time to do anything himself).