Treat this like any serious boss fight.
Try to buff as much as possible, if you can at this point.
Make sure to have plenty of items on you that will deal with the Blind that he can easily inflict on your party.
Wail on him with Fire and Lightning based magic.
If two people in your party know healing and buffing magic, you should be set.
I found Locke, Edgar, and Sabin were a great combination for this fight.
This fight is made easier if you've got Edgar's final weapon, that will deal massive damage on Ultros.
This combined with Sabin's heavy hitting Blitz moves and Locke's agility and overall bamfness should prove effective.
Hope that helps.