In my opinion(and the way I beat him) when it comes to who is in your party I used Zidane,Freya,Stiener,and either Dagger or Eiko. I know not alot of people used Freya in there final parties, I don’t know why she has a great command. For Freya you should have her use Drangons crest this will do 9999 damage every time. For Stiener his Shock(or was it Climhazzered??) should do the same(9999). I had Zidane steal or attack. And Dagger or Eiko can heal.
For a different strategy the offical strategy guid tells you to have everyone epuip with the insomniac ability and have Quina use Night at the end of each round. If everything goes as planned this should put Deathguise to sleep and keep him that way.
Hope this helps.:)