Ice Titan
Fought in the Olympia Stadium, the Ice Titan isn't as hard a fight as it is long. His attacks follow in a pattern that is pretty consistent. Fighting him may take some time, but it isn't a total challenge. For a start, you must have the Guard ability equipped. When the battle begins, the Ice Titan will release a stream of icicles at you. Time your Guard well to deflect the icicles back at it to deal some damage. Never cast Aero magic in this battle as it will cause the icicles to become too big to deflect. When you have dealt enough damage from the deflected icicles, the Ice Titan will fall to its knees.
Quickly run up to it and whack its head as much as you can (e.g. using Ars Arcanum or Ragnarok) before it stands up. One of its attacks involves it hitting the ground and icicles spring out from it. Either run or Dodge Roll to avoid damage. Another attack involves it raining down energy balls. Again, run or dodge roll. When its health bar reaches a certain stage, it will use a more powerful attack: It will deep-freeze you for quite a hefty damage. Experienced players can roughly predict when it will use this attack. But if possible, run to the side of the stadium (where the stairs are) to avoid its freeze.
Phantom
Fought in Neverland, if we come back there near the end of the game (you will notice that the Clock Tower is inaccessible from the World Map). Head to the room where Tinkerbell is and talk to her to get to the Clock Tower. Peter Pan must be in the party, because only he can understand the pixie. The battle is hard for two reasons: summons cannot be used and the Phantom uses "Doom" that kills your party members one by one when the clock strikes 12 (and no, Cure magic doesn't work on them). To stop the countdown and buy some time, cast Stop magic on the clock face (but it works temporarily).
Cast "Aeroga" to reduce damage & to Bounce off the Phamtom's "Death" Spell on Sora which he casts every time he gets in front of the Clock Tower (have MP Rage since Phantom deals quite a lot of damage, you can easily recover MP fast), since MP is crucial in this battle. The Phantom has a "heart" that glows in different colours, indicating what magic you should cast on it.
- Red → Fire,
- Yellow → Thunder,
- Blue → Blizzard,
- White → Physical attack
Now, you can either rely on Donald to Cure the party (if lucky enough to strike the Phantom with the right magic)when your MP is low. This battle requires patience and having to keep a look-out on the clock face to stop it from moving when needed. You will need mega-Elixirs to help you win
Kurt Zisa
Fought in Agrabah, if you come back there near the end of the game. You must go to Aladdin's room and talk to the Flying Carpet to fight this boss. When the battle begins, it holds out energy balls in its hands. Destroy them in order to attack Kurt Zisa (destroying the balls also releases HP orbs). It will fall and you can then attack its head (as soon as possible), before it starts swaying its head erratically.
Sometimes, it will cast a spell which prevents you from using magic, so you will have to depend on your items to heal. It can also cast a protective barrier which can be broken by magic (thunder is recommended since it can reach Kurt Zisa from a distance and do decent damage).
Again, when the shield is broken, you can attack its head again. At a certain stage, it will spin around like a nutcase. Try to dodge roll it or if you manage to summon Tinker bell before your magic was locked (to heal you) or cast Aero magic.
Sephiroth
Fought in Olympus Coliseum in the Platinum Cup. Sephiroth is seen as the hardest boss in Kingdom Hearts (original version, not Final Mix). Sephiroth has an invisible health bar, so don't be worried that you aren't doing any damage to him. When you miss a strike on him, he will cast "Flare" around himself, damaging you twice. You can use "Strike Raid" to protect yourself from both damage instances, but timing is crucial. After his HP meter turns pink, he will teleport away and cast "Death" by saying "Sin Harvest Angel", which will automatically take you out once the halo and wings appear on you. The ability "Second Chance" is a must here, since it will reduce HP to 1 and MP to 0 instead of a auto K.O. You can cancel the Spell by hitting him (try using "Super Glide"). If he is out of your reach, run as far as possible and use an elixir (note: you have very little time to do so). After his HP meter turns orange, he starts using "Omnislash" (a chain of flying attacks), so try using "Guard" randomly or "Dodge Roll". After his HP meter turns green, he starts using Meteor + Ultima. By standing next to him, you won’t get hit by Ultima. (You cannot damage him during the animation) and attacks you with orbs of dark energy.
Once you are familiar with his timing and what his attacks look like, it's actually not that hard of a battle.
Xemnas/Enigmatic Man (Final Mix only)
The hardest boss in Final Mix. His attacking style is the same as in Kingdom Hearts II. Heis found in the room in Hollow Bastion where you previously fought Maleficent Dragon.