Paralyse and Poison, and if available, Sleep on a friend medal. Just use plain taps for the first round and get everything to trigger. If the skills don't trigger, simply hard-reset the game (force the app to close and re-open it) to try again until they do all trigger. Cerberus will miss his initial turn, his buffs will be worn off and you'll be able to fight him properly now.
To simply kill him, focus on single-target medals. Aqua helps a lot if you obtained that from the event. If not, poison will still help you out a lot. Simply use the strongest medals you have, buff them properly if you can and you'll be able to take at least one bar of health away per round, if not more.