To begin the operation to take down Sin, select that option on the airship’s navigation list, then head to the deck once the cutscene is over. There will be more cutscenes on the deck, then it’s time for the first boss fight.
BOSS - Left Fin
The battle against Sin’s Left Sin works similarly to the one with Evrae, as you are on the airship and can command Cid to move in or move out. In fact, you will start the battle away from the Left Fin, so you should try to move in on the first turn you can. The Left Fin has a habit of doing nothing sometimes on its turns, but this seems to be determined on the number of times it has been attacked, as well as if it’s close or far away. When the Fin does attack it will usually do a physical one that hits the entire party for around 1,000 damage or so.
By far, the most annoying move in the Fin’s arsenal is called Negation, which doesn’t have a set pattern and can seemingly be random. Not only does this remove any buffs on your party (except for Auto-Life), it will also remove Armor/Mental Break from the Fin itself. Note that if it is afflicted with Mental Break, it can also use Negation when far away, but this won’t hit the party at all. The last move that the Left Fin does requires it to charge up some energy first, so if you see that, get ready for Gravija on the next turn. This skill will shave off 75% of your party’s current HP, but only while up close.
You should be using Armor Break on the Left Fin, as it is armored and physical attacks are not going to be effective without Piercing on the weapon. Other than that, there’s nothing really too special about the Left Fin. Gravija cannot kill you and in fact, the only thing that can is the physical attack. So, just make sure you heal up after Gravija (a Mega-Potion will work wonders) and you should be fine.
BOSS - Right Fin
After destroying Sin’s Left Fin, you will have to fight the Right Fin next. This is the exact same boss fight as the Left Fin, so resort to that strategy in dealing with it. The only change is that the Right Fin will attack more physically, with the slight increase of the chance to see Negation.
BOSS - Sinspawn Genais and Sin’s Core
The next boss is a bit of two-for-one, with Sinspawn Genais and Sin’s Core. Sinspawn Genais is similar to Sinspawn Geneaux in appearance, except it will begin the battle out of its shell. Any magic sent towards the Core will be intercepted by Genais, so you will need to dispose of it before you can go after the Core. While out and about, Genais will use Venom twice on a single party member (inflicts Poison), then use Thrashing, which hits all party members. If you use magic on Genais, then it will counter with Waterga. The Core will do nothing while Genais is out of its shell, so don’t worry about it.
However, once you drop Genais’ HP below 50%, then it will retreat into its shell and any attack on it will cause it to counter with Cura. While in its shell, Genais will attack with Sigh on each of its turns, which is magical damage and hits the entire party, while also inflicting Darkness. While Genais is in its shell, the Core will begin charging and using Gravija, which has the same effect as the Left/Right Fins. Note that if you’re not outdamaging the Curas from Genais, then it will come out of its shell again when it goes above 12,000 HP. This will also make the battle a lot harder, but you should be more than strong enough to outdamage the heals.
Both Genais and Sin’s Core are armored enemies, but only the Core is susceptible to Armor Break. Genais is weak to Fire, so Firaga should deal some good damage to it, despite the Waterga counter. Once you defeat Genais, the Core will be vulnerable to melee attacks. You will sometimes receive a counter from the Core, in the form of a tier 1 elemental spell that hits the entire party. You might also see Negation being used here, sometimes more than once in a row. Overall, the Core and Genais is not a difficult boss, once you can manage to defeat Genais.
You will return to the airship after the fight with Sin’s Core and Sinspawn Genais. Before heading for the final boss in this gauntlet, you will want to do a little bit of preparation. To be on the safe side, you will want Stoneproof (Stone Ward can also work) on three active party members, as well as Confuse Ward/Proof. Head to the deck when you’re ready and speak to Yuna for a scene, then return to the deck to fight Sin once more.
BOSS - Overdrive Sin
The fight against this form of Sin is more of a race than anything else. At the beginning of the battle, Sin’s first three turns will be spent moving closer to the airship, while using nine more times to slowly open its mouth. Once the mouth is fully open and the Overdrive gauge is full, Sin will use Giga-Graviton to cause a game over. There is no way to avoid this or block it, so you will need to defeat Sin here before that happens. While Sin is far away, you can use magic, as well as Wakka’s physical attack, to deal damage.
Also, if you happen to have any Overdrives full for Aeons, you can also use those. Once Sin is up close enough, you will really need to pull out all the stops. Before anything else, you will want to use Armor Break, as Sin is armored. If you didn’t cast Hastega/Haste on your main party, then do that to give them more turns. Note that Sin does have a counter in this bout, usually after striking it a certain number of times. That counter is an attack that hits the entire party for damage, while also possibly inflicting them with an ailment. There are three variations of this attack, with the ailments being either Petrification, Zombie or Confusion.
Zombie is nothing to worry about, as Sin won’t use healing spells. Confusion and Petrification, though, are bigger problems, so hopefully you either have some protection against them or quickly use Remedies to get rid of the ailments. Since it is possible to save before this fight and actually visit other locations, it’s not a huge problem if you can’t bring down Sin here. If you can’t, then that just means you need more damage.
Following the scene after winning, you will land and be able to enter Sin now.