Hidden Bosses - Guide for Kingdom Hearts

There are four hidden bosses in Kingdom Hearts. One gives you a very useful magic 
spell, while the other three are for prestige only. You can fight the first one by 
visiting Tinkerbell in the Ship's Cabin after finishing Neverland. 

 This guy, simply called Phantom, has a Death Sentence-like spell that kills off 
your party after a certain amount of time-cast Stop on a clock face to delay it 
temporarily. To hurt the boss, note his current color: cast Thunder if yellow, Fire 
if red, Blizzard if blue, and use regular physical attacks if he's anything else. 

Beating him earns you Stopga, which will likely be your last "ga" spell. Once you've 
got them all, Donald gets one more rod weapon. 

 The other three optional bosses are all exclusive to the U.S. version of Kingdom 
Hearts. Beating them takes extreme patience-don't expect to put a dent in any of 
them until you're at least level 60. 

The first one, Sephiroth, is unlocked by finishing the Hercules Cup in the Olympus 
Coliseum. He's essentially a staggeringly powerful version of Cloud...with a dash of 
scathing magic on the side. Equip Second Chance, keep Aero on at all times, and use 
every offensive ability in the book. When in doubt, level up some more. 

You can fight the Ice Titan after finishing the Hades Cup. His only vulnerable spot 
is his face, and the only way you can hit it is by blocking the ice shards he throws 
at you, deflecting them back upward. It's not nearly as difficult as it looks, 
making this battle a great way to stock up on experience if you're aiming for level 
99. 

 Finally, Kurt Zisa can be accessed by going to Aladdin's House after finishing 
Hollow Bastion and talking to his carpet. Kurt has two forms: one where he throws 
slashing blades at you and another where he floats around in a bubble and drops 
stationary fireballs. The first form's the worst-keep moving at all times to avoid 
getting hit-but the second one's easy if you can make him run into the fireballs he 
throws out.