Here's a vid I took to help out people that are having trouble with Hydra it's not a guide since I don't really wanna spoil the whole fight but maybe you can pick up on the things we were doing and just learn from it and GL!!! And thanks to Chiphead Xiv to figuring out the fight along with Mikari Cait.
http://www.youtube.com/watch?v=hlZPEYq_KOU