make some breach lower resistance and greater malsion spell triggers, and then cast a time stop... wait a minute he's immune to time stop
just use your best weapons and heal alot. i beat him the first time i tried, and not a single one of my characters lost more than 1/2 their life. i thought he was pretty easy.
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whats funny is I beat it in SOA and i went back to talk to the preists that run the place, and i told them i defeated the imprisoned one, and then the game was like no valid response, and then the conversation ended. i tried it a couple times, and the same thing happened, i left it and went back in TOB and it gave me some XP and all the knights complained to me.