Hydra is easy. Just try and lure its water bullets into killing the golems, then zigzag as you approach it and when your close, just hold a block whenever he is going to attack and attack his heads when they are in the ground. When you get down to the last few heads it will seem like the heads are just out of reach when they hit the ground, but just be patient and they will land near you soon enough. Also beware of the ledge in the water, if you fall you will die, it is quite easy to see however. I shat myself when I first saw him, but thought yolo and he was actually very easy.
Also you probably have your favourite weapons already but I highly recommend the Halberd. After 4 playthroughs, its just so much stronger and quicker than anything and I am yet to find a better weapon. Once you get a boss to stall, equip it double hand for a few hits and revert back for some amazing damage. Definitely leave Valley of Drakes, even just before the final boss its tough. Next on the list after the Hydra would be Sens Fortress. You can go further into the garden or get the crest from the blacksmith but its not necessary and will be much easier after you are done with Anor Londo. Sens fortress is more of a platforming area imo and once you learn how to deal with the Snake dudes, its quite simply. Also you will get the lighting spear there which is probably the best weapon at that point in the game.