I beat Ozma with Zidane, Steiner, Freya, and Quina, but I like to fight necron with Zidane, Steiner, Eiko, and Quina.

Quina is a surprisingly good "toolbox" is you get enough blue magic. I just Berserk Zidane with the Ultima Weapon and have him go nuts. Eiko is a good healer. Steiner has Shock. I would use Freya instead of Steiner, but for me, Steiner is a lot faster, has about 2k more hp, and isn't worthless when he has no MP.