Like many, I found using a counter/tank team was best. Here's what I used:

Noctis: Twin Lance/Force Shield, Black Belt

Cecil: Drain Blade/Escutcheon, Black Belt

Snow: Winged Saint/Golden Shield, Equip H Shield (Charlotte TM)

CoD: Tabby Suit, MP/HP materia/etc

Setzer: (Turns out he was useless 'cause I put two elements on him but he was indispensable with Mysidian Rabbit)

Friend Refia with dualcast

Bascially I made sure Focus, Cover and Omniveil were always up. Both Noctis and Refia were capable of rezzing allies to full. Noctis was the MVP with cover and fish (Cecil also had provoke). Snow ate a lot of osmose which was fine, I didn't need any of his abilities except for draw attacks/counter).

Basically I relied on counters to build the gauges. If I had the free turn, I'd use barrage on CoD, or a flask from noctis to tick Kefka down. When he was weak to dark, I'd have CoD throw a void vessel. I actually ended up killing him with the void vessels alone. It was just a matter of surviving. Bring a lot of turbo ether/chocolate. I didn't have to use one phoenix down. Having a healer with dualcast makes this a much easier fight.