I found Cerberus isn't like the other boss fights (Medusa, Cyclops, Minotaur) where you can sit back and strafe at range and pepper him with arrows. I kept getting baked by his ranged fire ball shots and dying before I could do any kind of damage.
I switched up to close quarters and took him out on my first attempt. I mainly used the Poison ability for my sword and dagger attacks and the Heal ability as needed. I didn't use any kind of over power attacks. I have most of my damage in Assassin and didn't bother shifting over to more Warrior damage.
Cerberus starts out with a ranged fireball shot that he shoots one at a time from each head. Get in close and he won't use that attack. When you are up in his business, he will do 2 different AOE attacks. If you are in front, he will lunge forward at you. If you are at the side or behind, he will rear up and do a ground pound. Both attacks can be dodged by holding X to dodge/roll away. Then jump in and hack and slash.
Once you knock down half of his health, he bursts into flames and the ranged attack changes to 3 fireballs at the same time (one from each head) so you really want to be in close. It also adds a sweeping flame thrower attack to the front. It was pretty easy to just stay to the side and dodge/roll away from the ground pounds.
Cerberus does try to jump away every once in a while but there is time to charge back in. If he happened to set me on fire, I could roll and heal then get back in to hack away.