Thanks for giving it so many tries!
There are many ways to win this battle, but here are a few things should give you a significant advantage.
1. Be sure to extract the Snake Stone, which gives you the "Bind" skill. Use this on the Cerberus to inflict the ATK Down status effect.
2. Keep some "Small Holy Balms" in your inventory to remove negative status effects
3. Be sure to use Angus' "Sludge Bomb" skill when needed. This can delay the Cerberus' next turn so that you have a chance to heal party members with low HP before it attacks.
4. Lastly, the Cerberus is weak to Fire elemental damage, so use Ysabel's "Fireball" spell as often as you can.
Hope this helps! Good luck!