There are three seperate battles to defeateing the hydra. I am assumeing that you are at the final hydra battle with the two small hydras and the final boss (if not let me know and I can help guide you through the first two battles). Deeating the two small hydras is exactly like how you defeated the second hydra on the ship deck. Once they fall down like they are defeated you must jump up on the boxes to the left or right of the hydra, depending on which on you defeat. Go the entire way to the top and jump on the platform with the anchor-hook on it to slam it through the creature's neck, thereby pinning it down. Work fast or the main hydra boss will heal it's smaller allies. Do the same with the other small hydra. Note that the lesser hydras will pull you away from one another with sucking winds, don't try to run away but rather run toward the opposite hydra and after you get so close it will stop pulling you in at which point you can run back and attack the first hydra. Don't be afraid to use your Posiden's Rage magic on the lesser hydra's, they will give you magic orbs, but you can still over do it so stick to physical attacks as much as possible. After beating the lesser hydras you will beable to fight the main hydra. Climb to the top and prepare for the first true boss battle. The main hydra has a very limited set of attacks, it mainly bits and roars. When it roars the force of the air will push you of the platform, you will not fall, but rather you will be forced to hang on the edge near where you climbed up. Holding in the block button will permit you to stay on the platform and sneak in a couple attacks on the boss. Beware of it's bite attack, it deals massive damage, and it looks similare to the roar. Stay near the rope you climbed up and you'll avoid the attack every time. After it bites take advantage and attack as much as possible, usually after you attack it will roar but sometimes it attacks again. If you cannot excape the attack use you magic attack to avoid damage and greatly damage the hydra, but use your magic as resoursefully as you did with the lesser hydras. After a bit the hydra's head will fall to the platform and you will be promped to hit O. Do so and contenue to tap O rapidly to damage the hydra or it will heal. Repeat the this three times to finally defeat the hydra.

After you hit him so many times his head will fall on the plat form a circle will appear push the O button to grab him then keep pushing it fast and you will slam his head against the pole repeat it and he will die but you have to beat the small heads first hit them one at a time until the head falls then run up the wooden boxes get on the small plat form it will fall and stab him in the head