Hydra

The Hydra is a multi-headed monster that is found in Kingdom Hearts II. It is one of the bosses at Olympus Coliseum. It originally appeared in the Disney movie Hercules.

Kingdom Hearts II
Hades sent the Hydra to destroy the Olympus Coliseum and kill Hercules. Hercules had to fight it in the Coliseum and believed he had defeated it, but it was in fact was merely playing dead and started to destroy the Coliseum. The Hydra met its match when Sora, Donald, and Goofy as well as Phil, Pegasus, and Megara came in to help out. However, the aftermath of the battle was rather unfavorable - the Coliseum was destroyed and Hercules lost his hero's glow, making him later fall prey to the plans of Hades.

Strategy
The battle begins as Sora attacks the main head for a while. Then a Reaction Command called Vanquish appears - doing so will allow Sora to remove its head. It puts its head in the sand and three heads sprout from the surrounding area. With Phil's help you can immobilize the heads with a Reaction Command called Phil One-Two and then follow it up with Urninator. You then must defeat and Vanquish the heads before they awake.

The Hydra then pulls its neck out of the ground and shows that it has seven heads. While fighting the seven heads, you can use Pegasus to fly up to the heads to strike them with a Reaction Command called 'Pegasus Run'. Once it is out of HP, you can use the Vanquish command again. If you ever run out of HP, MP, or Drive, Megara is found at the outside the arena itself, hiding on the very left side of the arena. Simply run around the area, and will toss you what you need.

Hydra is capable of summoning dark orbs, using lightning, and using blue fire similar to Hades. At the end of the battle you get Thunder and after the cutscene, Hero's Crest. If you die, don't worry - King Mickey will be able to help you.

Reaction Commands
Phil One-Two: When the Hydra has it's head in the ground, follow Phil's advise and "Get up on the Hydra's back!". When you do this, this reaction command is available. Phil will throw an urn into the air, and Sora will leap into the air.

Urninator: After Phil One-Two, use this Reaction Command. Sora will hit the urn downwards with his Keyblade, and it will smash after it lands on the Hydra's back, stunning it for a short time.

Pegasus Run: When this Reaction Command is activated, Sora will mount Pegasus, and they will fly at the Hydra's heads.

Attack: During Pegasus Run, use this Reaction Command to attack the Hydra's heads with the Keyblade.

Vanquish: When you have depleted the Hydra's health bar to zero, it will slump over. Use this Reaction Command, and Sora will slice quickly through the Hydra's necks, defeating it.

Trivia

 * Like the movie, the Japanese version of the game has the Hydra spew green blood when a head is cut off. In the American version this was replaced with purple smoke, though the green blood is still visible during the first cutscene of the Hydra.
 * During the battle against the Hydra, you have the most character support of any battle. Donald and Goofy in your party, Phil tossing the pots to stun the Hydra, Meg giving you HP, MP and Drive orbs and Pegasus allowing you to use the Pegasus Run reaction command.