I think the best tips/advices I got when trying to fight Kangxx (as well as other liches) is that 1 what Memnoch mentioned (avoid at all costs imprisonment -- a bad thing
2 remember to stall and stay away 3 characters in Baldur's gate (including Kangxx and you) have a limited supply of spells! pretty much helpful by just hitting and running... sooner or later any NPC will run out of spells. Just make sure your hitter/runner has boots of speed or Hasted. 4 also, I found out that waiting too long to take on Kangxx does NOT actually make the fight easier... he gets tougher too. I guess that goes for most other fights with NPCs. Is there an automatic adjustment thing behind the scenes of the game that pits a suitable strength for your opponents depending on your level?!? Just curious.