Stage 1: All Three Heads-
Use the Cestus (fully upgraded if possible) to attack the Cerberus. Always be dodging and attacking from behind on its tail. Make sure you are always dodging or the little vermin children of his will catch up to you and explode on you. Pummel on his back end with the Cestus, and if you dodge carefully enough constantly, you should be able to get the Circle prompt to come up with hardly any of your health taken away.
Step 2: Two Heads Left-
Select the Gorgon Flash in your souls inventory, and quickly switch to the Claws of Hades, because one of those pole-dancing acrobat swordsmen will spawn. They are almost impossible to take down with that Cerberus running about, spewing his death children, so the only way I found able to kill him was to Gorgon Flash him and destroy him as a statue quickly. If you do that, you can focus on the Cerberus once again, following the same strategy as Step 1. Switch back to Cestus and pummel the Cerberus. Hopefully, you still have at least half your health by the time the second Circle prompt appears.
Step 3: Only One Head Left-
This is where it gets very tricky, and where luck and careful positioning played in quite carefully. I wasn't even intending on this to happen, but it worked out perfectly and I wasn't sure why I didn't think of it before.
You only have enough magic for one more Gorgon Flash, yet there are two new pole-dancing acrobat swordsmen that have spawned while the one-headed Cerberus flounders about. Well, like I said, it's nearly impossible to kill them. So you have to Gorgon Flash them. But you only have enough for one!
Well out of dumb luck, I used mine when they were both very close to each other. They both got frozen by the Gorgon Flash and I smashed them both in one hit.
Now all you need to do is focus on the Cerberus with the Cestus again for one last home stretch. Keep dodging carefully and attacking from behind. You don't want to die here after all the hard work you just had. Use Rage of Sparta if you absolutely must. One more Circle prompt, and the battle is over