I have to say that Necron was the hardest boss for me, for some reason, whenever he did Grand Cross, it always leveled my characters. Ozma was easy to me; I used Freya, Zidane, Eiko, and Steiner and made Freya use Dragon's Crest, Steiner use Shock, Eiko heal, and Zidane steal or use Dark Matter. Since I had 99 Pheonix Downs, Phoenix always revived my characters. Freya's Dragon Crest technique does the same damage even when she's in Mini status and with just the right weapon it does 9999 always. I got this strategy off a website but I can't remember which one, but I've never lost!