Part of the Killing in the Name of... Quest.
The portal which leads to the boss is in depths of Goroma. The entrance of the cave is here.
Follow the patches to reach boss:
Kerberos is in north of last cave.
Damage Taken From Elements
Goroma (look at spoiler for more info).
Kerberos fights to the death.
Their melee attack is considerable much weaker than their wave attacks (which are fatal if combined) so it's best to block it while standing diagonal. Magic Walls, Wild Growth or Energy Wall Runes can be used to help you to staying diagonal and preventing you from the wave attacks. These creatures are very strong so don't underestimate them!
Paladins Should use Assassin Stars and cast Divine Missiles while wearing some Stone Skin Amulets. At lower levels use Energy Rings, stay at meele range, don´t run and heal with Great Spirit Potions or Strong Mana Potions trying to stay at full mana all the time.
Mages can try to block them without mana shield at level 200+ with some amount of luck while being quick with your healing spells, while shooting Sudden Death runes. Also it's recommended to wear some Stone Skin Amulets to reduce the Physical Damage, alternatively Might Rings.
Kerberos is weak for Ice Damage and Holy Damage, so use ice or holybased attacks.