Its all about the equiptment and skills, when I fought him i did this

Zidane: Chimera Armlet (guard against Holy), Ninja Gear (absorb Shadow)
Vivir: Pumice Piece (absorbs Holy and Shadow)
Steiner: Demon's Mail (absorb Shadow), Ribbon (reduce Holy by 50%)
Ekio: Pumice Piece (absorbs Holy and Shadow)

everyone had Auto-Haste, Auto-Regen, Auto-Potion, Auto-Life, and the HP/MP modifier skills

above level 65 and he was cake

And on a side note if you beat ozma before fighting hades you hear an amusing line before the battle