Instructions
The initial step is to decimate the Hydra's monster hordes. Then press "R2" to open the hull that holds this mythological water beast.
When you come across the beast for the first time, begin by utilizing your quick attack combos until it pulls away from you and flaps her ears, both actions being a sign of an attack.
Hold "L1" until this attack ends and the Hydra returns to its original stance.
Repeat these steps until a "CIRCLE" symbol appears over the Hydra's head. Run toward the symbol; press the corresponding button and the other sequence of buttons that will follow. This will kill the Hydra, for now.
Make your way through the rest of the ship. There will be men running around, flailing their arms. They are innocuous, but the birds that are pecking at them are not. Kill the flying beasts, and if you are running low on health you can slash the men until they'll throw out some health orbs.
Once you're back on deck, keep your eyes open because the Hydra will literally pop up without warning.
Use quick attacks once again, but this time focus on the monster's neck.
Prepare yourself for the Hydra's initial attack. It will slam its head on the deck in this sequence: left, right, then toward the camera. Dodge these movements in that order.
Continue using your quick attacks until the Hydra swoops its head down and attempts to grab you in its mouth. There is no way to avoid this attack. The best you can do is minimize the damage by quickly hitting the grapple button, which is "R1," until you are dropped.
Avoid the Hydra's attacks and continue to use you quick attacks until the Hydra has finally vanquished once and for all.
Regain some depleted health by lifting a few nearby doors to uncover some red health orbs.