How To Defeat Hydra for A Relic Reborn Quest.
From what I can see, the mages and ranged DD need to stand as far back on the opposite side of the room as possible until there barley out of range but still in range. Reason is because it does Radiant Breath or whatever it's called and it's a devastating AOE if it hits you. So that means the main tank needs to pull the Hydra to one side. As far as the close DD, Radiant Breath won't hit you if you stand on its sides of the boss, so keep that in mind. The main tank needs to run inside the boss when he casts the breath, and I mean actually inside, don't try running far away you won't make it. when it casts its other close range moves, the PLD and other close DDs need to dodge it, dont stand there with red circle or whatever under you and get hit by the attack. He is also gonna start casting ground AOE's by the people standing far away, all you have to do is dodge it. When the add appears, your second tank needs to aggro it and all DDs besides main tank needs to kill it ASAP.