Even if Daystar isn't strong enough, it would be good to use as many Sunrays as you can. On the old Forum, this guy posted a screenshot once of his fight with Kangaxx and he managed to do an incredible amount of damage using sunray (about 1000). Try to have a high level Viconia in your party or if you must Aerie, Sunray works better as you increase in levels. Have them memorise as many Sunrays and False Dawns as possible.
Do you still want to know where to find Kangaxx? There are three crypts spread over Athkatla (BEWARE THE TRAPS ON THE DOORS), in one you'll find a talking skull who'll give you the quest of looking for his other body parts. To find those, go tho the other tombs and prepare for a fight! Once you have the torso and the arms+legs, go to the floating skull again. You need weapons like the Vorpal blade, Staff of Rynn, Sling of Everard etc. to hit him, also protection from magic scrolls and a berserker would be very useful as well.
Yes dragon slaying weapons work on Big Red, you'll find a bunch of them in his lair so use those!
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Melusine, Archbabe of the OHF and the LH
Your voice is ambrosia