The first optional boss that you’ll want to fight is Phantom. Head off to Neverland and notice how you cannot land at the Clock Tower. Instead land in the Ship: Cabin and swap out Goofy for Peter Pan. Talk to Tinker Bell and agree to go with her to investigate the Clock Tower. After a short scene you’ll be thrown into battle with Phantom.
Did you know? Phantom is based on Lord Ombra from "Peter and the Shadow Thieves"
BOSS - Phantom
This fight can be difficult so you’ll need some preparation. Gear up with MP increasing Accessories and Keyblade, also stock up on Ethers and Elixirs just to be safe.
The key to beating Phantom is to cast Stopra on the clock at the start of the battle. This is because Phantom casts Doom on one of your allies. The counter starts at 12 and if it reaches 0 that ally loses it’s heart, but using Stopra on the clock also stops the Doom countdown.
With the most crucial part of the battle out of the way, you need to move on to how to damage Phantom. There is an orb at the base of Phantom that changes colors which is a reflection of what attack to use. Red means Fire, Blue means Blizzard, Yellow means Thunder, and White means a Physical attack. As you can see, having items and MP regen abilities will be helpful. Also, mapping at least two of these spells to your quick keys will help.
Phantom will take three hits before the orb breaks and try to claw you. The Heartless then moves towards the clock and begins to cast a poisonous energy ball. If you get hit by this you’ll take five ticks of damage and it can be deadly. The simplest way to avoid this is position yourself around the corner of Phantom when he casts the spell so it hits the wall. This is also a good time to recast Stopra on the clock. You’ll want to do this about every other time Phantom casts his energy ball to avoid losing a team member.
The battle will loop like this until you beat Phantom so use Ethers/Elixirs as you need to keep your MP high enough to cast spells.
After the battle you obtain the power of time, Stopga .
The next optional boss you’ll want to tackle is Kurt Zisa. Head to Agrabah and land at Aladdin’s House where you’ll find Carpet jumping around and pointing frantically. Talk to him and he’ll take you into the desert.
Did you know? Kurt Zisa is named after Kurt Zisa of Medford, New York who won Square’s "KINGDOM HEARTS: Name-In-Game" contest that was held a few months prior to the North American release of the original game.
BOSS - Kurt Zisa
This is a three phase battle and you’ll want to have Aeroga set as a quick cast. As soon as the battle starts cast Aeroga because Kurt Zisa will use Silencega, an ability that will cancel all spell casting until the dark orbs in his hands are destroyed. Kurt will only use Physical attacks during this phase which consist of dashes and swings of his weapons, called Khopeshes. It shouldn’t be too hard to get in and hit the orbs with Aeroga aiding in some of the damage as well as mitigation.
Once the orbs have been destroyed the second phase starts. Kurt Zisa breaks down in this phase and uses it’s head to stretch out like a snake to attack. The attack is weak and shouldn’t do much; you can heal the damage anyways since magic is no longer sealed. Take this opportunity to really lay it on Kurt Zisa. Seriously go all out and get as much damage as you can in. If you can take down at least half of his health you’re in great shape. Once the head retracts and Kurt Zisa stands up this phase is over and the third phase begins.
Kurt will put up a shield and float around the arena summoning tornadoes and fireballs. The trick is to deal damage to the shield in the form of magic. Thunder is good but Blizzard is best if you can get all of the flakes to the hit the shield – that requires patience. Hitting the fire back at Kurt is also effective at knocking the shield down. Physical attacks will cause clear orbs to drop so you can refill the Charge Gauge which is useful.
When the shield goes down Kurt Zisa will revert to phase two and then back to phase one. When phase one starts again Kurt Zisa will twirl around the battlefield and you’ll need to Dodge Roll or Glide past the attacks, or else you face massive damage. By the third time you enter phase two you should be able to defeat Kurt Zisa.
You learn Zantetsuken for defeating Kurt Zisa and receive Ansem’s Report 11 .
The final boss you’ll want to face is listed in the Journal simply as "?" and given the title of Unknown. Having played the other games in the series you can deduce who this Nobody is, but it won’t be included here. You can face the Unknown by heading to the Castle Chapel in Hollow Bastion. Step into the dark portal to start a scene that leads into a battle with this foe.
Who? The achievement "He Who Doesn’t Exist" lists it as "Defeat the Mysterious Man in Hollow Bastion" though the journal lists him as Unknown. Playing through Re: Chain of Memories and watching 358/2 Days should give plenty of insight as to who this figure truly is.
BOSS - Unknown
Start off by settings your shortcuts to Graviga, Aeroga, and Curaga. Believe it or not, this foe is effected by Graviga. Not in the standard way of course, but it will bring him to his knees allowing you to hack away.
Start the battle by summoning Tinker Bell and casting Aeroga. Lock on and get ready to Dodge Roll some lightning fields. Unknown will toss around balls of lightning that hit the ground and spread into a sphere of pain so keep away. With those avoided, you can start laying combos into this Mysterious Man. You will take damage, but it’s easy heal through with Aeroga up and Tinker Bell might just do the trick. If you had a bunch of items left over you could always stock up Donald and Goofy with Elixir and Megalixirs so you are in tip-top shape.
Once you deal 300 points of damage, Unknown will start to teleport around. After another 300 points he’ll begin to summon ethereal blades that he will toss at Sora. Dodge Roll this attack and start using Strike Raid to deal damage while becoming invulnerable to the lightning damage.
Once the Unknown is down to 600 health he’ll start using a technique that messes with your commands. Triangle and your top three options will alternate between Shock and Release very quickly, you need to hit the right one. It is best to just smash the Cross button and hit up then down in hopes of landing on Release quickly. The Shock doesn’t deal much damage so you want to be quick about getting released.
During this time you’ll want to start using Graviga before Unknown teleports and you’ll bring him to his knees. This is great for healing or tacking on some additional damage. If the going gets tough, use Strike Raid and Sonic Blade to avoid all the damage and use Elixirs to refill your MP. Gravity Break is a great finisher for this fight so you can add that as well.
For defeating Unknown you’ll acquire an Exp Necklace and Ansem’s Report 13 as a reward for a tough battle. Reading through this report should shed some light on the Unknown’s identity.