Wait, isn't that Zemus/Zeromus you're fighting, not Golbez?
After he changes form, the crystal is no longer any use. You're going to need to keep using your strongest attacks, and stay on top of your health. Like any good FF final battle, it's a matter of endurance. There are attacks that will wipe out a lot of your team, and you're going to have to make sure you keep healing them and reviving them. If your whole team is at full-strength and gets wiped out by one attack, that's a good sign that you need to go back and raise your experience level first -- it's also a sign that you probably ran away from too many normal battles before this point.