Ah, Faceman has given me an idea.
HARM
If you have a Mage/Cleric, and assuming you casted 3-4 Lower Resistances beforehand, get him/her to cast Time Stop from a scroll, cast Harm and whack Jon Irenicus, reducing him to
1 Hp.
If your Mage/Cleric still has time during the Time Stop, proceed to spit on Jon Irenicus or poke him.
Remember, all hits made during a Time Stop are guaranteed hits.
I am in doubt though, since I recall Jon Irenicus being immuned to the effects of Time Stop in the Nine Hells. If he is not, all the better.