Tactic 1: Aiiim...Headshot!Edit
Now you only need to wait for the Hydra to attack, then shoot whatever ranged weapon you have in the Hydras open mouth. But keep in mind you have to stay in the Hydras reach (10-25 blocks), otherwise none of your ranged weapons will be able to harm it.
Now just keep doing this until the Hydra is dead. (Be aware of its bite, its a potential one-hit-death for you even with good Armor).
Tactic 2: Spider.......hook??!Edit
Requirements: the Spiderhook from killing Shelob and a high damage melee weapon (if possible a Cleaver from Tinkers' Construct) plus higly enchanted Cloud Boots, so you dont take falldamage. Armor better than Iron is recommendet.
Hook on one of the Heads and reel in until you are basically in the Head, then keep hitting it till the Head is dead. Keep doing this until the Hydra is dead.
-When inside a Head the Hydra cant attack you.