The Ultimate Battle-
Pick a party consisting of your best three characters. I prefer Squall, Rinoa, and Irvine. Give Squall and Rinoa each some Aura spells. Make sure Rinoa has Eden, and that Irvine has the item command and at least ten Holy Wars. These can be modified from Gilgamesh's card. Start the battle by having Squall cast Aura on himself, Rinoa cast Aura on Irvine, and after that's been done, have Irvine use a Holy War. From there, use Squall and Irvine's limit breaks, and have Rinoa summon Eden. Once the Aura wears off, have Squall attack normally and have Irvine wait for the Holy War to run out. When this happens, have Squall recast Aura on himself, Rinoa cast Aura on Irvine only if her turn is loaded, and Irvine throw another Holy War. Speed is the key. If all goes well, you should be able to dispatch him within six Holy Wars. This is assuming that Squall has Lionheart. If he doesn't, you're pretty much... well, screwed, so get that! It's the key to your success in this, the last part of the game.

You must fight... Rinoa?!
Adel alone would be quite the pushover. However, she decides to experiment with a new look: Rinoa melded into her torso. That makes this battle somewhat difficult, as you don't want to hurt Rinoa, so your GFs are pretty much useless. Begin by casting Shell on Rinoa. You might want to cast it on your own party members as well. If your physical attacks are weak, cast Regen on Rinoa as well, you'll most likely be in for a long ride. Use physical attacks on Adel, however avoid limit breaks. Most of the time these target randomly or on multiple targets, so it could hurt or, if strong enough, maybe even kill Rinoa. About every five turns, throw a hi-potion on Rinoa. Keep your HPs up, you never know when Adel might pull out Meteor or Ultima, both of which can be fatal if they catch you at the right moment. Continue to keep your people stable, including Rinoa, keep Shell on Rinoa to cut down on Adel's life draws, and the once mighty sorceress should crumble soon enough.