SPOILERS:
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Through out the final battle, when things get ugly, Jon will teleport away to different part of the Nine Hell map. Usually the direction of the lighting when he teleports will give away his position.
To kill the slayer, you will need to take out the demons ASAP, they are nothing but distractions for you. Make sure a mage or cleric in your party has true seeing on since Jon loves his simulacrum or mislead. Your mages should have sequenced lots of magic missiles (single magic missile, minor spell sequencer, spell sequencer, and spell trigger). But before you unleash any magic missiles, make sure you lower his resistance at least twice. Keep on wearing him down with fighters, it is a pretty long fight due to his regenerations. Whenever you got him to badly injured, unload some spell sequenced magic missiles on him, he will go down pretty soon. (com'on it is a 6 on 1 fight, assuming you have a full party)
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Long live H22A1!!!!!