Hydra is the Eleventh boss you will face and its element is Fire/Water so it is weak against Water/Air and it is strong against Spirit/Fire. You can face it in the Normal stage of Blazing Tides or at the Summoning Stone. If you beat it 10 times, you will get the achievement Head Hunter. It drops Hydra Scales which can be used to craft Hydra Hunter's Mail or Crimson Scales which can be used to craft Dragonflame Armor. Beating the Hydra on Epic stage gives access to the Sunken Carrack.