by Eric Zellers

Perhaps the hardest part of Final Fantasy IX is the secret boss, Ozma.Ozma is a real nightmare and is nearly impossible to defeat, even at high levels. You can defeat him, but it really requires pre-planning. You can't just run into battle unprepared. Here I will state what I believe is the best strategy for defeating him (and yes, I have beat him before)
The most important thing is what characters you will use. I'm sure that you could beat Ozma with any combination of characters, provided they are strong enough, but I recommend a basic team of Zidane, Dagger, Steiner, and Vivi.
The next most important thing is to have the right equipment. The characters need to have some of the most powerful items in the game.
Zidane should be equipped with the Ultima Weapon. He needs to have Ninja Gear as his armor. And he needs to have a Ribbon as his Add-on. His headgear and armlet are not especially important so just equip him with items that raise Magic Defense.
Dagger should have either the Whale Whisker or Tiger Racket as her weapon (although that's not actually that important). Her headgear, armlet, and armor need only be whatever raises her Magic and Magic Defense the most. Most importantly, she needs to have a Pumice Piece as her Add-on.
Steiner should have the Ragnarok equipped (or Excalibur II if you actually bothered with that!) His helmet and Armlet should be Genji Helmet and Genji Gloves. His armor should be Demon Mail (its not that great of armor, but there's a reason for it.) And his Add-on needs to be a Ribbon.
Vivi should have the Mace of Zeus equipped as a weapon. His headgear and armlet just need to be something that raise his Magic and Magic Defense. He should have the Robe of Lords as armor. And finally he needs a Pumice Piece as an Add-on.
Next, abilities are very important. This includes regular and equipped abilities.
Zidane really only needs to be able to use Grand Lethal. As for equipped abilities he needs HP+10% and HP+20%, Auto-Regen, High Tide, Counter (and Eye 4 Eye with it), Auto-Potion, and as many status ailment prevention abilities as possible. You probably won't be able to use ALL of these, so just prioritize (status prevention and High Tide are most important.)
Dagger needs to know Bahamut and Curaga (Don't get Ark from Hades because you need the Pumice Pieces!) Her equipped abilities need to be, Half-MP, Auto-Regen, Boost, and as many status ailment prevention abilities as possible. Feel free to throw in Auto-Potion or High Tide if there are any left-over points.
Steiner only needs to know Shock as a regular ability. For equipped abilities, he should have the same ones as Zidane, except I recommend not wasting points on High Tide for Steiner.
Vivi needs to know the Doomsday spell. For abilities, he should have High Tide, Auto-Regen, Auto-Potion, Half-MP (Ok, I didn't use it, but it couldn't hurt), and as many status ailment prevention abilities as you can get.
Now for the actual battle. Before hand, you should have a full stock of Hi-potions, Remedies, Ethers, and Elixirs. You also need to have found all nine of the friendly monsters. You might want to get your characters' Trance meters nearly full before this battle. You need to get a Gold Chocobo and go the the Chocobo's Air Garden. On the island toward the back, you can find an eidolon grave. Ignore the moogle's warnings and go straight into the fight. If you didn't find all 9 of the friendly monsters monsters, then Ozma will probably kill you immediately. Don't hesitate for a second! Just have Zidane attack (and use Grand Lethal when in Trance), Dagger use Bahamut (boosted and with 99 Garnets in you inventory), Steiner use Shock, and Vivi use Doomsday (this is the reason for the Pumice Pieces and the Demon Mail and Ninja Gear). Ozma uses some nasty attacks of his own. His Meteor spell will probably kill you in one hit, but his worst attack is Curse. It inflicts multiple status ailments on everyone and deals a lot of damage at the same time (this is the reason for the Ribbons and status preventing abilities). When he uses Curse, immediately use Remedies on everyone, unless they were miraculously not horribly messed up by the spell. Curse will probably put Mini on your characters, so it wouldn't be a good idea to even try to attack in that condition. If you are lucky, Ozma may do something stupid like cast Doomsday, which will heal your characters, and will deal 9999 damage to himself. Most importantly, you need to be persistent with Ozma. You will probably not beat him the first time. You need to fight him several times to get a feel for his strategies. I suggest all your characters be level 76 or higher, but do not have your characters be any level that is a multiple of 5 since Ozma uses Lv5 Death.
For beating Ozma, you will be rewarded with a Pumice and a Dark Matter. You will also receive a "strategy guide" (It's just a key item that gives a small hint about the game). You will also get a large amount of EXP, AP, and Gil.