Have one character with the following materia:

  • Link HP Absorb with Knights of the Round materia
  • Use W-Summon materia to summon Hades first to stun him and then Knights of the Round (unless it's leveled and you can summon it twice)
  • Mime that over and over and over again so that you keep casting Hades and Knights of the Round

You can have Final Attack + Life if you want, but if it gets to that point you won't be able to use the Mime strategy anymore. Also, take off Counter Attack so you don't accidentally hit him in between Mimes. You might want to have the other two characters dead as he'll use that whirlsand thing on you.