What helped me with EVERY boss in FF8 was to write down their attacks, then prepare for the fight by junctioning accordingly. So for Cerberus, have all your characters junctioned so that they'll regenerate when hit with Cerberus's magic attacks. I think he uses quake and lightning, right? Anyway, just write down what he does, and when he kills you, go around drawing the spells you need to be optimally junctioned. That was one of the things I really liked about FF8, if a boss was giving you crap, you could just go re-arrange your stats at will without needing to level up.