Tip
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The reason why it seems so hard is that Irenicus in Slayer mode has an obscenely high regeneration rate( RR). So you need to deal a lot of damage to compensate. Poison/Acid weapons will be useful here, since they slow down his RR. Use Arrows of Biting( you have them right? o_O?), Melf's Acid Arrow, to warm up.
After that, throw everything but the kitchen sink at him. Remember, you outnumber him, you will win, eventually.
Cheesy tip
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One word. ----> Web
If you possess the Ring of Freedom, or if Keldorn is in your party, then cast multiple Web spells on Irenicus and send in your forever freedom fighters to engage an entangled Slayer. Get the rest to either soften him up with spells or missile weapons.
You can try Entangle too, but Web has a higher chance to trap because of the -2 to save bonus.
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If not a Sorcerer, then a Paladin.
