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Spell Immunity: Abjuration is the best thing to use when fighting demi-liches or other evil denizens who have the ignorant habit of casting imprisonment non-stop.

The best tactic for Kangaxx is to use one of your party members in that specific fight. Where you can kill his Lich form with the entire party, make sure the others run away, while your mage (equiped with a +4 weapon) casts Spell Immunity: Abjuration on him-/herself. Keep hacking, use RotR to deliver some more damage. Then you should eventually be fine.

Question: Do demi-liches see through invisibility? Otherwise cast improved invisibility on yourself. Imprisonment is a target-based spell, so it can't be cast if you're (improved) invisible.
NOTE: Improved Invisible, because that invisibility lasts even when you attack. Must be dispelled first.