Trivarium: To start off you will need 2 tanks, both of which will be main tanking Hydra -- the reason being is that Hydra will cast an almost instant spell called Trivarium. Trivarium is a frontal AOE breath (no AOE indicator) that will do a tremendous damage to a single tank; however the damage can be split among 2 tanks if they stack on top of eachother. Hydra will also have other AOE cone abilities; however they can be easily side stepped since there will be an AOE indicators for those abilities.
Adds: A wyvern add will periodically join the fight, and it will be the DPS job to burn the add down as quickly as possible. The add will cast a buff on hydra increasing Hydra's damage. There will be no off tanking here since the off tank needs to be with the main tank -- so stunning the add will be highly beneficial here.
Ball of Ice and Fire: Hydra will periodically cast AOE patches of ice and fire at a random player. These patches of ice and fire will stay on the floor for quite some time, so be sure to watch your step. Also note, it's imperative to not fight in the middle of the room for Hydra runs to the middle of the room to cast Fear itself, which I'll explain next.
Fear Itself: When Hydra is about to cast Fear Itself he will run to the middle of the room. Fear itself is a huge AOE that will do tremendous damage to all players. To avoid it, you must stack on top of Hydra in the middle, and hopefully you will not have patches of fire or ice in the middle. Note, be sure to especially avoid patches of ice on the floor since they can slow your movement speed -- you won't have much time to get to the middle once Hydra starts casting Fear itself.