You don't really need Cerberus at all. I think after I got him I only used him on the last fight with Edea, and that was it. I get it whenever I play because it allows you to junction more stuff to your characters. As for beating Cerberus, I use Carbuncle to the max, in that fight. The poor little thing takes a beating in that fight.

(BTW, Gullwing, your sig rocks)