I don't have a full-blown strategy in mind for fighting Cerb, but here are a few things that may be helpful:

Have a few Dispel and Haste spells if you can. Haste in general is a good spell to have, especially when dealing with fast, poweful physical opponents. I'd suggest drawing and casting Triple on a party member with Haste in his/her magic stock, then Triple-cast Haste on everyone.

I urge you to have Dispel in your inventory for the times when Cerberus casts Triple on himself. Depending on how your characters are junctioned, three consecutive high level spells can be devastating. Dispel Double and Triple status straight away.

Pay attention to your Vit stat. If your party members have a low physical defense, Cerb's physical attacks (especially those delivered by his tail) can take a good chunk of your characters' HP.

That's all I got for ya. Anyone else feel free to modify/add/ignore/whathaveyou, as you see fit.