Ruby weapon, until I found the entirely ultra-cheap method (well, this one takes a lot of time, but it is still very cheap once you get the materia for it) of killing all party members but for Cloud (or Yuffie), giving him whatever weapons and armor allows for the most materia (because you're going to want as many master materia as you can get at the end of this battle with the massive AP available). Then you make sure the following mandatory materia and combinations are equipped: W/Summon, MPturbo-Master summon, HP absorb-Knights of the Round, and a mastered command-counter-mime. That way, you just w/summon Knights of the Round, and with the combinations I just listed, you'll gain 9999 HP and each knight's attack in the summon should do well over 8000 points of damage. Then when Ruby weapon counters, you're chosen charachter will counter by miming you're last action, at which point I suggest making a sandwich or going on a short walk, because after about 15 minutes Ruby weapon will be quite dead, and your chosen charachter will still be at full health.

As for Ozma, all you have to do is find all the friendly monsters and give them what they want, after that Ozma's fairly easy since you'll then be able to attack it physically.