I equipped an Opal Ring on my main attacker and set the other two to support. As long as you don't dispel reflect then he puts up the magick palling less often. Then I nailed him with Scathe again and again. Don't have everyone cast Scathe at once though. They can only cast black magick one at a time for some reason but support magick can be cast all at once. If you have only one character use scathe then you'll get in a lot more hits, especially if that character has haste and a high magick stat. I couldn't defeat him until my characters had levels in the 90s. If you're not there then go and get Ultima. Deep in the Great Crystal is a great place to level.