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.
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!