I just tried killing Kangaxx last night. The average level of my party members was 12.
For the first form, Jan's 6 traps hit him pretty badly and Keldorn's Ring of Ram put him to "Near Death" state. I quickly got Jan to cast Melf's Minute Meteors (he memorised 4 sets, which was barely enough) while my PC cast Spell Immunity:Abjuration. The second form emerged.
Kangaxx then directed all Imprisonment spells at my PC (who had the Staff of Magi by that time), while Jan pelted the meteors. My PC then cast minute meteors too, since I realised it seemed to do more damage than the staff (COULD ANYONE PLS CONFIRM THIS?). The 2 mages took turns to cast the meteors as they ran out quickly and I was starting to worry that the abjuration spell would run out soon, so I activated the spell trap function of the staff and finally sent Keldorn in (didn't send him in earlier as I was afraid he would be imprisoned and since I was only at Chapter 3 of the story, there are no freedom scrolls available).
It was a pretty tense 15 seconds as I watched my PC and Keldorn hit the dying Kangaxx with the only +5 weapons we had. To my immense relief, Kangaxx finally collapsed. I'm thinking of rewarding Jan and my PC with both rings, though the rings would probably be more usefully employed with melee fighters
I don't know if I will ever bother reloading to see how tough it will be to kill Kangaxx without traps at all, but I'm sure it'll be a challenge...