You should use the same strategy used to defet Omega Weapon against Ultimecia. However, you may want to take out the use of Holy Wars to make your party invincible, because Rinoa can sometimes get the same basic thing when using Angie. I believe the attack was called the Invincible Moon. If nothing else, I suggest you have a lot of the Aura spell available, so that you can use Limit Breaks every turn. It also helps to have Squall's Lion Heart, because it causes around 100,000 points of damage when the Lion Heart Limit Break is used.

You may have noticed that the last couple of forms of Ultimecia can kill a GF in a single shot. Don't use your GFs unless you're protected by Holy War, Invincible Moon, or Hero. These make your characters and your GFs invicible to any attack. Outside of that, you must keep everyone's HP up and ensure that everyone is as fast as possible. If you boost everyone's speed through Junctioning and then cast Haste on the entire party, you can attack nonstop. Also, cast Triple on everyone (or at least summon Cerberus) at the start of the fight. This should make it a lot easier to heal everyone and to cast Aura on your entire party.

Also, don't try to equip everyone before the battle with Ultimecia. You'll spread yourself too thin if you equip absolutely everyone with GFs. Equip only the three you want to use the most, and then let everyone else get killed and absorbed. Soon enough, you'll have your main party of three and then you can really unload on her.

To emphasize once again, most of this is the same stuff used against Omega Weapon. If you follow this information and the strategy for Omega Weapon, you shouldn't have any trouble.

Remember when you named Squall's ring? Whatever name you give the ring is the name Griever will have. In fact, you'll notice that the profile on the ring is Griever's profile.