Well time to finish the game.
First off. If you oversouled every fiend in the game. You can talk to Shinra examine his bestiary and get The End Garment Grid. Gives Break Damage Limit and Finale. Finale is an attack that does maxed out damage unless you escaped from battle.
Now the Farplane Abyss. This place consist of a rather vexing puzzle. Due to the security system you can't pass unless you break through it. You can choose to do it in 2 ways. Either brute force it or actually solve it.
The Brute Force method leads you to a boss fight wish one of the toughest bosses in game. Azi Dahaka. While it's Guardian Beast yes. It's no pushover unless you're maxed out. This guy is capable of inflicting Curse and Itchy Status. Which effectively stops you from taking any kind of action. Itchy prevents actions unless you change dresspheres and Curse prevents changing dresspheres. So don't take it lightly.
The later method consist of you going around the area activating switches. Each switch will play piano notes that you kinda need to memorize. Playing them in the correct order after you discover what they are will open the path.
Either method you'll have to do it 3 times before you reach the end. Now for the final battle(s).
Vegnagun consists of 4 battles. Rather easy unless you're fighting this at an extremely low level with no abilities. The tail doesn't do much but attack the party periodically. The Leg has cores that can't be attacked except with ranged attacks but you don't need to worry about them. The Torso is pretty much a pushover as well. The fight that matters is the Head. Shuyin will be in a dire rush to fire Vegnagun and you need to shut it down before he does so. If he fires it. Game Over...and in Majora's Mask Style. But you have PLENTY of time to stop him. I'd say you have a good 15 minutes to finish this. Taking it down you need to take out the other area's first. This will reveal Vegnagun's weakspot. During the fight Vegnagun will pelt you with annoying attacks that get stronger as its HP drops. They also cause various status effects. Once it's dead time to finish things once and for all with Not Tidus.
Shuyin is literally a clone of Tidus. All of his attacks are Tidus's overdrives but with different names. (Too bad your Tidus can't learn 2 of them). They do kinda hurt but it's nothing you can't handle. Silence the crybaby and end the game.
Depending on how well you did. Determines what ending is available. I honestly don't know how it's supposed to really work. Cause I got 100% on my other file and got the same ending despite doing everything right. If you press X the dead fayth will approach Yuna asking you if you want to see "him" again. Yeah "Him". Whatever you answer gives you your ending. I'm not showing that cause Square Enix will ride me for doing so.
(Note: My 100% Ending had Yuna telling Tidus it's better if we see other people. But I've been told that's not what supposed to happen. I don't know what the deal is. I don't really care at this point.)