For CHAOS, I swapped armor a bit, transferring the Ribbon and ProRing from Soul the red mage to Biff the fighter. Both fighters needed the protection for this fight, since if they die, I'm done for. The Red Mages don't need to do anything but cast FAST.

Biff got the Masmune; as the second character, he's less likely to die from physical attacks than the leader

Round one: 430 damage from Biff (unFASTed, even) and 50 from Tank. CHAOS did only a physical attack to my leader, who, not being a Berserker, weathered it in stride.

Round two, the fasted fighters just attacked, while Hart FASTed himself too, and the other RM cast CUR3 on my leader. CHAOS came with ICE3, severely denting Soul, while the fighters delivered 200 and 350 damage. I'm more than halfway there, and he's several rounds away from CUR4.

Round three, fight fight fight CUR3 for Tank again. Soul's low on hit points, but he's completely dispensable now. And LIT3 from CHAOS almost did so.

I hoped for the kill in round four, but both fighters landed no crits, scoring only about 50 damage each. CHAOS finally came with his infamous CRACK, plonking Soul, but that's okay, he can rest in peace. I don't see any way that CHAOS can possibly kill both fighters now.

Round five brought another physical attack from CHAOS on my leader, who returned the strike...

VICTORY!

Final Fantasy beaten on a single party life and no level grinding! Folks, I've wanted to pull this off for about the last sixteen years.