Leviathan

Part of the Killing in the Name of... Quest.

Heals as quickly as a Hydra and stronger, also runs away in deep red health.

To all vocations: Keep yourself diagonal and wear a Protection Amulet or Leviathan's Amulet since most of the damage you will receive comes from their melee.
Treat it like a strong Sea Serpent with more hit points, stronger attacks, and faster healing. You have 10 minutes to kill and loot it or you will be kicked out.
Knights: kill it using Berserk, Fierce Berserk and Whirlwind Throw when it starts to run. It is highly recommended to cast Blood Rage and Annihilation, then combine with Berserk, Fierce Berserk, Brutal Strike and Front Sweep.
Paladins: Paladins should use Assassin Stars and use your best potions to heal.