Cerberus

Cerberus is is a three-headed hellhound who guards the Underworld. He first appeared as Hades's loyal pet in Disney's Hercules, as guard dog to the Underworld. Cerberus serves as a boss in Kingdom Hearts, Kingdom Hearts II, and Kingdom Hearts coded.

Kingdom Hearts
''Guardian of the entrance of the Underworld and Hades's loyal servant. Herc defeated him once, but Hades has brought him back to fight Sora and his friends.''

He first stood watch for Hades in "Hercules" (1997).

Kingdom Hearts II
Hercules (1997)

Guardian of the entrance to the Underworld and Hades' loyal "pet."

Hercules, Sora, Donald, and Goofy teamed up to defeat him last time, but Hades brought him back -- and he's as dangerous as ever.

Kingdom Hearts
In Kingdom Hearts, Cerberus acts as a back-up plan for Hades, if Cloud, whom Hades promised to help find Sephiroth for him, fails his task of eliminating Hercules and Sora.

When Sora and Cloud's battle comes to an end (regardless of who wins), Hades sees that Cloud is no longer of any use to him, and sends Cerberus in. Cerberus quickly crushes Cloud beneath its front paw when his back is turned, and is about to attack the others, but is held off by Hercules, allowing Phil, Sora, Donald, and Goofy to escape.

Cerberus has Hercules, who is carrying Cloud, trapped, but Sora comes to help, returning the favor and allowing Hercules to escape. Sora manages to defeat Cerberus, and his efforts reward him the status of "Junior Heroes". However, as Sora leaves, Hercules mentions to Phil that he had already worn Cerberus down by the time Sora intervened.

Cerberus can be fought again in full power at the Hades Cup, having four health bars instead of two (thus proving the fact that Hercules had worn it down). However, since the first time you may not have obtained Cure, it might seem easier the second time you fight it.

Kingdom Hearts II
In Kingdom Hearts II, as Sora, Donald, Goofy, and Auron are escaping from the Underworld, Hades sends Cerberus to stop them. Donald and Goofy escape as Auron fights off the creature. Sora returns to aid him, but ends up separated from Donald and Goofy, leaving him and Auron to fight the monster by themselves. The two then manage to get away, and Cerberus is trapped behind the doors as they close, but not before Sora, Donald, and Goofy taunt him by making faces. Cerberus has its own cup in the Underworld, being the final opponent of the Cerberus Cup in a timed fight.

Kingdom Hearts coded
Data Cerberus appears as a threat to Hercules, Cloud and Sora. He is the source of the Bug Block corruption in Olympus Coliseum.

Strategies
It is important to be able to climb on top of Cerberus's back, in order to avoid its Dark Breath attack and his Fireballs. Alternatively, you may use Dodge Roll to avoid Cerberus' Dark Breath attack, or even deft running. Use the Guard ability to deflect its fireballs. (Note: going under Cerberus' back when it uses Dark Breath will not work.) Use its back and the stairs as places to heal yourself, which is important to do often. Be sure to have a load of items like Ethers and Potions, or be sure to equip a Keyblade and items that give you at least 10 MP. Do not rely on Donald for healing, as he dies quickly, especially in the first battle. Be sure to lock on and attack the heads on the side, not the middle head. Use Dodge Roll and/or Glide (or alternatively, Superglide) often to get away from his Bite. After several attempts to bite, it will stand on two legs and roar in the air. When he does this run away from it, or be ready to jump, as it is about to use a powerful shockwave. Keep attacking the heads with your Keyblade and use a combination of Fire and Thunder magic to finish it off. While attacking the side heads, you can cast Aeroga on yourself to increase the damage. Cerberus is by no means an easy opponent, but at the same time, it should not cause the player too much trouble. The player is only forced to face it once, but does so with Auron alone, meaning no drives. Also note that this fight is held in the Underworld, meaning drives are off limits until the Olympus Stone is obtained. The first thing Cerberus will (usually) do is jump right in front of Sora, so be ready to deliver an aerial combo. Sora will be thankful Auron is here, as his Bushido is immensely useful; although Sora should try to hit two heads with it. The Star Seeker Keyblade is recommended, as it's aerial boost will be helpful while performing aerial combos. Cerberus isn't keen for a chat though, as he often swivels around leaving you nothing but his back, preventing damage. For those who use the Keyblade more then magic, just lock onto a head and start using combos, and Bushido whenever possible. If the user prefers a magical route, use thunder on one head until you have almost no MP left, then use Bushido. You are often entered into a contest of survival when Cerberus tries to eat you, when this happens, use the reaction commands to make things easier, and this contest looks brilliant as well. Cerberus has just over two bars of HP, which isn't that large, although much bigger then Sora's HP. And if this precious HP gets low, stick to the edges and use quick dash to avoid Cerberus when it jumps over to you.
 * Kingdom Hearts
 * Kingdom Hearts II

In the Cerberus and Cerberus Paradox cups, you fight it alone, although you can still use drives. Sora can use Valor Form to hammer some sense into him, or play it safe and shoot magic bullets at it with Wisdom Form. Master Form is the most recommended form though, for some reason it seems built to fight Cerberus. All of the abilities Master Form has equipped will help Sora defeat Cerberus. Of course Final Form is the best, and Sora should use the blade if it has it, but that isn't very likely.

Since the Cerberus and Cerberus Paradox cups are timed battles, you should be wary of the time constraint. If you have Final Form, use it. Every single movement is an attack in itself, and this should buy you time to keep on mashing. Do not compromise unless you have managed to take all the lesser enemies out within seconds.

Trivia

 * Although most of the world bosses in Kingdom Hearts: Chain of Memories are the same as in Kingdom Hearts, Cerberus is replaced by Hades, who was an optional boss in Kingdom Hearts.