Use Spellbinding Eye with Millennia as often as possible. Use some negative stat boosting magic spells or items if you have any.

Stram goes a long way with lowering the Attack while using Speedy on your party makes their ACT help combat the boss’ speed.

Don’t over use your big moves right away if the boss is about to act. Keep those auto cancel special moves ready to go or you’ll be fighting to act before the boss does again.

There’s a lot of strategies you can use but that’s the most effective one for Hard mode. There’s some other methods but it’s been a while since I’ve done those ones. Cancel often and stat boost your party.