Actually you win the whole Pumice from Ozma. The other pumice piece you get from defeating Ark in Oielvert.
I just thrashed ozma last night, (YES I know the game has been around ages but I haven't had much time to play lately) and this is how I did it. First you REALLY should do the side quest where you feed gems to all the friendly monsters (lots of places on the net tell you the locations) so you can attack ozma physically.
My party - Zidane, Steiner, Quina, Dagger/Eiko
MAKE SURE NOBODY IS SUSCEPTIBLE TO LV5 DEATH!
My party were all high levels (Zidane 99, the rest 81)
Equip them with items that protect against or absorb Shadow and Holy. Have auto-regen, antibody, clear headed.
Have Quina get rid of Ozma's MP with Magic Hammer. (will take 3-4 hits) He'll drain your MP, but it doesn't give him much to work with, and wastes his moves. If you've captured lots of frogs (like me) Quina can do 9999 damage with frog drop. He/she can also use Angel Snack when you get curse cast on you.
Use Climhazzard with Stiener, and attack with Zidane. Zidane is pretty useless in this battle unless he goes into trance. That didn't happen for me, Ozma died too soon. Eiko/Dagger is best used healing (use the R1 button to select whole party to heal instead of just one), reviving those who get KO'd, and restoring MP with ethers and such. You can use Eidolons if you wish but I didn't bother.
The battle took about 10 - 15 minutes but I never once felt in danger of being annihlated. The reason it takes so long is because Ozma likes to cast Curaga on himself.
Yeah. Note. I don't think there IS a 'quick' way to kill Ozma. You just gotta be patient.
Hope that helps.
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(Ugh, I haven't played any videogames since last Wednesday cause I've been so busy revising! Then all I got time to do afterwards is come here, read email and sleep. 