All right, my tactics for Improved Irenicus (keep in mind it was a while ago, so it may not quite be my actual tactic, but it should work)
Party consisted of:
me (fighter/thief, dual-wielding Sachuu and CF+5)
custom made char(fighter/druid, dual-wielding Water's Talon and Belm (this combination rocks: with Imp. Haste, you can have 10 attacks/round and with every attack you're poisoning
)
Solafein (dual-wielding (yes again, I like it) Daystar+4 and Equalizer
Imoen
and Tashia (with Robe of Vecna)
All characters have about 4m-5m XP, pretty high level, I admit.
Also I had a lot of healing potions AND (this turned out to be crucial) invisibility potions, as well as Improved Items installed and making use of it (four rings of preservation+2, nice)
Before the battle I did a standard buff-up (stoneskin/ironskin, mirror image, imp. haste/haste, blur, fireshields) and summoned some mordies and a planetar, which I had standing between the Orb and the Dragon spawning places. Then I opened the door and Jon would appear, giving his speech and all that, nd wouls leave me with a couple of shadows. The mordies take care of that, while Imoen and Tashia would cast Time-Stop.
Tashia would finish first (Robe of Vecna) and with IA, she hits the Dragon with Lower Resistance two times and NUKES him (ADHW, Comet, Dragon's Breath, Skull Traps). Also try to send a couple to Selfish, and if you can, to Greed (in the case of Greed, Magic Missile does wonders). Time Stop ends. Solafein, wearing the Cloak of Balduran, has more than 100% MR and targets the Orb of Fear with the planetar.
Time Stop of Imoen starts. She targets Greed as well as Selfish with about everything she gots. Time Stop ends, Greed near Death and Selfish badly injured. Imoen and Tashia finish Greed off and start working on Selfish, who doesn't last long (he is targeting Sola also).
Meanwhile, the mordies have whiped the shadows and have started on Wrath. Fighter/thief has his swords back, he and fighter/druid target Wrath also. Main char uses Assasination, which is really helpful: he doesn't hit always, but when he does he does tremendous damage. Druid poisons Wrath a bit with her scimitars. Wrath is in the meantime slaughtering the mordies, but it takes some time I use. Sola is ready with Orb, has Critical Strike, so helps against Wrath, who finally falls. Now you have a couple of spare time. I used these to Improved Haste my main character.
Now phase 2. Slayer appears, and I am teleported away from my equipment. No loss, as since I'm still hasted, I can easily pick it up and put my boots of speed on. Now I'm faster than the Slayer! For some reason, the shadows which have appeared start attacking my party members, who can't do anything. So I first waste the shadows, than attack the Slayer, which is actually quite easy (I suppose Sachuu helps...). Be sure to have some healing potions, but basically I was able to defeat him in pure melee.
Phase 3. Tricky. Some may consider this cheese, but when the conservation ends I gave everybody invisibility potions, which they drank. Now here comes the nice part: unlike the slayer, Final Jon can't see through invisibility and has no way of dispelling it. YES! Now seperate your main character from the rest of your party. Jon will follow you, but not attack you. This is a good time to slay the rest of the shadows with another party member, just keep out of sight of Jon (if he's in the way, move your PC: Jon will follow). Have him/her return to invisibility. Now WAIT until Jon's protections are somewhat gone, most importantly his invisibility.
Now it's the time for action. I had my PC, my fighte'/druid and Sola attack him (Sola went in first so that Jon targets him with Sola's 100% MR). Imoen and Tashia would try to dispel his spell protections and then his combat protections. Whenever Jon would cast Protection from Magical weapons and I was not able to dispel it (too many other protections) or when badly wounded I would drink inivisibility potions and wait. Finally, I had the Smite from Sola left, so I used that when he used Improved Mantle. He would lay still so I could do some damage, but what finally got him was the Backslash from Sachuu and the poisoning from Water's Talon.
Hope you can make something out of it.
P.S. what really sucked was that after the first success, I did not have the ToB CD at hand so I had to do it again! It was a bit easier then, though.