If I am not mistaken, Jon regenerates 3 hitpoints per second whenever you fight him in hell. What you should do is to have your mages cast anti magic protection spells on him (breach, spell strike, rubyray reversal ...etc) and also have you cleric with true sight on. Then pound away with remain characters, keep fighting as this is a looooooooooooooong fight.
When you hurt him enough, he will warp to another part of the map and make sure you chase after him ASAP due to his hitpoint regeneration. Try not to sacrifice characters as they can really help you surround him and pound away.
Good luck.
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