I dunno whether this'll qualify as bad spoilers or not, so I'll put it under tag just in case.
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In my Yggdra save (my first), as soon as I got Yggdra to La Pucelle, she became so powerful that I was always able to use her to win against Gulcasa. In the last three fights against him in the game, I was able to send in Yggdra alone, and even though Gulcasa set up unions, he never had a chance. Basically, once you get Yggdra (or, I suppose, other troops as well) up to level 17 or 18, and equip items to make up for whatever shortcomings they still have, all you have to do is go sit them next to him and use such wonderful cards as Crusade, Revolution, Dragon Killer, Ace Guard, Shield Barrier, and so on and so forth.
It's easiest with Yggdra though, because she's guaranteed MVP in at least two battles in the game. Ever since her class change, I was almost always able to send her in alone to fry any enemies I encountered.
But then again, I maxed her level by the middle of chapter 7. So that might be why. Anyway, Gulcasa never gave me any trouble after that, even though practically every battle against him up 'til then consisted of at least one retry and me yelling at him to "BE NICE, GUL-CHAN! I KNOW YOU KNOW HOW!"... see my LJ for details.