I recommend that you do not upgrade your weapons until the very end of the game. Your equipment is character specific, and often time if you max a weapon out, there's potentially a better weapon later on in the game.

Focus on leveling your accessories instead. All characters can use them, and they are useful up to the end of the game.

Try using a different party and paradigms. I always use Lightning Fang and Vanille for this fight, because Vanille is a strong white mage and Fang is a good Sentinel.