I only had two +3 weapons too (well I had some more in my inventory that I forgot I had until I beat him) and what I did was summon monsters to distract him from my party members. If he starts beating on a party member (leave everyone without +3 weapons out of the way) move them around until he picks on a summoned creature instead. Then move back and hack away (repeat as needed). And summon more creatures as the others die (preferably with the characters that don't have the +3 weapons). Hope that helped.
-Maj
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