The second, and probably more dangerous is the fireball attack. It acts much like TNT, and usually doesn't explode right after hitting the ground. These can be portaled away, deflected back with a gravity gun, thrown back, or just avoided.
The third attack is more of a bite, only used when you are extremley close to the hydra. Because of this, it is recommended to take it on ranged, as the bite will kill you instantly.
If you plan to engage it in melee combat, (Not recommended due to the bite, but it's your choice) it is suggested that you use Fire Protection potions, wear a set of Nano Armour, and wield a Nano Saber or Vajra. If possible, use Quantum Armour instead of Nano Armour to ensure an easy victory. These armors are not edible to the Hydra. Another armour is Power Armor, which allows you to hit its heads directly. During the fight it is advised to hug the base of its body. If its breathing fire at you (close-mid range), run in a circle. The fire is not instant and takes a moment to travel to its destination, giving you the time to dodge it. Once the fire stops, attack the head until its mouth closes (You can attack regardless of it attacking or not). Always keep up close to the body, as it'll allow you more time to attack, without compromising your security
If you prefer being ranged, try reflecting the fireballs its shooting like you would that of a ghast. The fireballs do a lot of damage to the Hydra (about 1/8th). If you're not as good about reflecting the fireballs, simply try hitting it's opened mouth with an enchanted Bow, or a Mining Laser. If using the laser, remember to use a battery pack with a lot of EU in it. Sentry turrets are destroyed to quickly to be of any use.
When the attacked head loses enough health it dies off, allowing two new ones sprout (The maximum amount of heads that can spawn is seven). Once the Hydra has reached seven heads, it can be killed for good once its health bar has been drained. Despite this being one entity, each head can attack a different person. Regardless of this, you will never find yourself targeted by two heads at the same time. Over time if the hydra remains undamaged it will regain some health so once you begin an encounter it is highly advised to keep an offensive mindset. Also lava will not harm it so the use of lava is more likely to endanger you.
Another tactic is to throw Holy Hand Grenades at it, which deal massive amounts of damage and are relatively cheap to make. It will generally take maybe 3-6 to kill it, however each crafting gives you 4 Holy Hand Grenades.
The Hydra drops Fiery Blood which is used to craft Fiery Ingots (more info on Fiery Ingots on the Fiery Blood page). The Hydra also drops a decorative item - a Hydra Trophy.
-The Twilight Forest