If you have taken the time to aquire the GF Odin, and Seifer has slit him in half, giglamesh will come in handy. He may destroy one of the forms of griever. Also, the strategy for the Omega Weapon doesn't help at all. Anyway there are a total of five forms to fight, and i figured that the best way to do it is to equip each member with one GF, and have them constantly heal and cast aura on Squall. Trust me. I've done it three times.The end is really long but awesome. Just make sure you can constantly use Renzokuken on the enemy and it will be no sweat. You can try any way you want, but i assure you this is the best way.