I still am at a loss as how to completely prevent irenicus from healing himself in the final battle in hell. I have tried ( in no particular order) ruby ray of reversal, breach, spellstrike, remove magic, pierce magic and dispel magic, and he JUST KEEPS HEALING HIMSELF.... is there a particular order these spells must be cast in order to make this a fair fight? I have been trying to solve this dilema for the past week and I am really in need of help... if someone would like to perhaps help me I would be very appreciative!!!!

His healing is not due to a spell-he has great regeneration while in Slayer. If you are talking about him actively healing himself in Human form, post otherwise.

Once you have his defenses knocked down you need to start hitting him with everything you have. And I do mean everything including the kitchen sink +3. Since he will regenerate quickly you have to hit him with heavy duty weapons and offensive spells. It never hurts to hit him with a couple lower resistance spells and then load up If he is spell sequencer with Melf’s Acid Arrow, Flame Arrow, Chromatic Orb or whatever your favorites ones are. My favorite tactic is to occupy him with Mordenkainen’s Sword and a couple other heavy duty summons.
As far as equipment goes; don’t be shy about using lightning wands on him, what are you saving the charges for anyway? Ring of the Ram is always good for knocking an opponent down, which is why I always have two of them.

thanks for the replies ... I got him down once but didnt quite finish him but I will ... thanks again.. my main question was can you keep slayer from regenerating and rehealing? and I believe the answer is... no!

You could try the Harm spell...this puts his hitpoints donw to 1 if the spell is successful. After that you shouldnt have much trouble to defeat him.
I think you need to lower his resistance a bit and cast a greater malison to stand a chance...even then I dont know if it will work but I do now it works well on Greater Wolfweres and trolls that too got a bitchy regeneration ;) And hey it even works on dragons some time which IMO is far tougher then Irenicus...

All I did was throw everything I had at him. I didn't even bother having my mages hang back in the ranks, I had them fighting at the front line too. The reason for that is, the more muscule you got pounding down on Irenicus, the more of a chance he'll have a spell casting failure.