The Final Battle
I don't want to spoil the battle flow for you, so this description may not be too clear, but you ought to be able to follow it.
For the first part of the battle, the sorceress is easy to get rid of. Cast Riple and Double on all your party, cast Regen, Protect and Haste on all your party. Cast Demis in triplicate and GF Diablos. Watch out for the Maelstrom attack, heal your party with Curagas and Esunas, or a Full Cure from Selphie (if she is in your party). She also uses: Holy, Dispel and Double.
For the next part of the battle, watch out for the Shockwave Pulsar attack. It is a magic attack, so casting Shell on all your party will help, but if all your party has about 9999HP, they will be alright. Just heal them quickly and carry on fighting. Use Auras and Limit Breaks for this battle. If you want to 'cheat' a little, cast Auras on triplicate (one on each party member) then use three Heroes, or a Holy War. For more information about Heroes and Holy Wars, click here. Use Limit Breaks and high powered magic spells like Ultima and Meteor. Doin't bother with your Guardian Forces. Three Ultimas or Three Meteors (You should have each party member with Triple or Double) will do more damage in less time.
For the next part of the battle, watch out for the Great Attractor attack. However, this attack is all bark and no bite. It's extrememly long to watch, and deals about 3000 damage maximum. Use Flares and Tornadoes in triplicate (preferably from a party member/s with Mag + 20%, 40% or 60% junctioned) and Limit Breaks if you wish. Don't bother to attack the Helixes.
For the last part of the battle, watch out for the Hell's judgement attack. It will reduce all your party to 1HP. Don't take the opportunity to use a round of Limit Breaks, it is too risky. As soon as you see this spell, there are a number of options for you:
1. Cast a Holy War, and your party wuill be invincible (and you can do some Limit Breaks without the need for an Aura spell, because the party will be on critical status)
2. Cast a Hero on one party member, and if the entire party is attacked, at least you will have a survivor to revive them (make sure this person has Life or Full-Life spells, or the Item command equipped and a supply of Phoenix Downs, Mega Phoenixs, or a Phoenix Pinion)
3. Cast a Curaga on everyone (and hope you aren't attacked by an Ultima spell; or have one party member with a high Mag Stat to cast the Curagas)
4. Cast a Curaga on one person (and hope you aren't attacked by an Ultima spell; or have one party member with a high Mag Stat to cast the Curaga)
5. Cast three Curagas on one person. This character will definately survive the next round of attacks and can heal/revive the othe rparty members, as needed.
6. Cast two Curagas on one person and one Curaga on another. The first character will survive the next round of attacks, and the second character should be able to as well. You will have less party members to revive if you are attacked.
7. If you have no Triple spells, cast a Double Curaga on someone. This character will survive the next round of attacks, as long as the party member casting the Curagas does not have a very low Mag Stat.
8. Use Quistis' White Wind Limit Break (if she is in your party). Have her learn this Limit Break by using a Whisper, which you can mug from Adamantoises (lower levels, use Level Down learnt by GF Tonberry) which are found on Long Horn Island, East of Deling.
9. Use Selphie's Full-Cure Limit Break (if she is in your party... and you better hope you get it to show up in time)
10. Use Rinoa's Limit Break and hope for Invincible Moon (but I wouldn't recommend doing this)
If you want, you can simple cast Heroes or a Holy War at the start of the battle, and then as needed, and you should spare yourself a lot of problems. Attack using Ultimas and Meteors in triplicate, and Limit Breaks too (use some Aura spells).