Cerberus is a level 318 three-headed hellhound who is weak to Crush. In order to fight Cerberus, you must have an active Slayer Task. You can access the teleport by opening the Teleportation Menu -> Bosses -> 2nd Page -> Cerberus
Set your weapon to Crush style, use your Super Combat/Overload, turn on Turmoil and Soulsplit, and start attacking Cerberus.
If you have the above setup, the fight will end in less than a minute, skipping Cerberus' ghost phase. The fight is AFK as long as you do enough DPS to outheal with SS!
If you don't beat Cerberus in time, you'll have to deal with the Ghost phase
Cerberus will say 'Aaaarrrrroooooo', and 3 ghosts will enter the arena, each with a different color representing their combat style. Flick the style's protection prayer as the ghosts hit you, to avoid damage.
Red = Melee, Green = Range, Blue = Magic
Cerberus can be one of the most AFK bosses on Arigon, provided you have a high DPS setup!