First of all, buy Scathe from the merchant in Nabudis. You'll need to deal magical damage half the time. Keep up the usual defensive magic. It's similar strategy to Zodiac: hit him with physical attacks when he puts up a magical paling and switch to magical attacks when he puts up a physical paling (I just set my leader's gambit to cast Scathe and have the follower's gambits set to follow his lead). Keep the party spread out, rinse and repeat, healing as necessary.

The convention that we have to fight the final villain personally is so strong it overrides other considerations.