Are you ready for the last chapter of the game?
Keep in mind that there are a ton of side quests and a lot of end game content that you can complete before you beat the game if you’d like. This is recommended if you want to get the full experience of Final Fantasy X so don’t rush to the end of the game too quickly! Use the following link to head back to the side quest:
The next few battles are against Sin itself. Sin has a very high amount of armor so you should make sure that Auron (or one of your other party members) has obtained the 
Armor Break ability. You can easily train up and level your party members by returning to the Zanarkand Ruins and battling fiends in that area.
Start off by selecting Sin on the “NavMap” aboard the Airship (Fahrenheit) and Cid will begin playing the Hymn of the Fayth. Travel back to the Cabin of the ship and ride the elevator up to get back to the exterior of the ship.
Note: The next two bosses (Left Fin and Right Fin of Sin) are the exact same fight that you have to complete twice in a row.
HP: 65,000
This battle is very similar to the fight against Evrae on the exterior of the Airship. The Fin will attack less frequently if you use the Trigger Commands to have Cid pull the Airship further away from Sin.
The Airship starts far away from Sin so you will have to use the “Move In” command before you can initiate any attacks. Before you move the airship closer though you should use the time in between to charge up your characters using Tidus’ 
Cheer ability, Rikku’s Luck ability, etc. Remember that you can use these abilities up to five times and the effects will stack.
Use 
Hastega on your party early on but be aware that the Sin can use a move called Negation which will remove any positive status effects from your party members. You will have to recast them afterwards.
Sin has a fairly high armor stat so you may wish to use Auron’s Armor Break ability to help speed things up. But be aware that Negation will also remove the Armor Break negative status ailment from Sin. Just reapply it if it is removed.
If you see the message “Core Gathers Energy” immediately use the Trigger Commands to pull the Airship away from Sin. Sin will be using Gravija shortly which will hit for approximately 3/4 of each character’s total HP.
Steal:
Mega-Potion (common)
Supreme Gem (rare)
Drops:
HP Sphere (common)
HP Sphere (x2) (rare)
In Game Description: Unleashes Gravija once the core is energized, reducing all characters’ HP by 3/4. To avoid this, get Cid to back off. Dispels with Negation and sometimes attacks all with a physical attack.
This fight has the exact same battle mechanics as the previous fight.
Steal:
X-Potion (common)
Shining Gem (rare)
Drops:
Lv. 3 Key Sphere (common)
Lv. 3 Key Sphere (x2) (rare)
In Game Description: After losing half its HP, it enters its shell on its next turn. If hit there, it recovers with Cura and exits the shell on its next turn. Protected from Gravija when inside the shell.
HP: 20,000
Start this fight off by destroying Sinspawn Genais which is the creature sitting in front of Sin’s core. Genais will absorb any magic that you try to use directly on Sin. This Sinspawn starts outside of its shell, unlike Sinspawn Geneaux which you fought at Kilika temple.
Sinspawn Genais is weak to Fire-based magic attacks. It will also retreat back into its shell once it goes below 50% of its HP and will begin casting Cura on itself. It does not have much HP though (20,000) so hammer away at it with regular attacks until you bring it down.
You can cast Slow or use Silence Buster on Genais in order to prevent it from casting Cura (as often).
Steal:
Star Curtain (common)
Shining Gem (rare)
Drops:
Return Sphere (common)
Return Sphere (x2) (rare)
This fight is exactly the same as the Left Fin and Right Fin fights. The only difference is that you will no longer be able to dodge Gravija attacks by moving the ship around and it will counterattack your regular attacks with low-level magic spells.
Your party members may be hit by the Poison status effect. Have Rikku use an 
Al Bhed Potion, have Yuna use Esuna or have any of your team members use an 
Antidote or a Remedy to remove it.
Steal:
Stamina Spring (x3) (common)
Stamina Spring (x4) (rare)
Drops:
MP Sphere (common)
MP Sphere (x2) (rare)
Level up your characters through the Sphere Grid and grab any items that you may have run low on. In preparation for the next fight, equip each of your characters with any armor they have that contains the following abilities:
Stone Ward or Stoneproof
Confuse Ward or Confuseproof
Zombie Ward or Zombieproof
Alternatively you can customize your equipment and add on the Auto-Med ability which will have your characters automatically use a Remedy if they are inflicted with a status ailment.
You can use 20 Remedy to customize your equipment and add the Auto-Med ability and then of course you will want to keep a stock pile of Remedy on hand so that your characters can use them mid-battle. This is not a requirement but it can make the battle much easier.
Make your way back up to the elevator and outside the ship to speak to Yuna and then again. Yuna will hint that she knows that Tidus is part of the dream of the Fayth and that there is more to his story than he is revealing.
Sin will approach the Airship again one more time for one last showdown…
In Game Description: Pulls the airship in while gathering all his strength to cast the ultimate terror, Giga-Graviton. Stop him before he wipes everyone out!
HP: 140,000
This is a timed battle. After approximately sixteen turns the fight will end as Sin will use its Overdrive ability, Giga-Graviton, which is an instant game over.
The charging bar above its head is an indicator of how close it is to using its Overdrive. Summoning an Aeon will not prevent death, which means you have to do 140,000 worth of damage before then.
Sin will start off too far away for you to hit it but it will slowly move closer (to within melee range) as the battle progresses. Use the first few turns to enhance your characters using Cheer, Luck, etc. and have Tidus cast Hastega on the party. Use either Lulu or Wakka in your party to do some damage even when Sin is at a distance.
You should have Auron use Armor Break once Sin gets close enough. Alternatively you can have Rikku use her Mix ability and have her Mix two items (such as two 
Power Spheres or two Ability Spheres creating a Frag Grenade) which can cause the same Armor Break status ailment.
Sin will begin attacking your party once it gets close enough to the ship so be prepared to deal the the Petrify, Confuse and Zombie status effects as they come. Good luck!
Steal:
Ether (common)
Supreme Gem (rare)
Drops:
Lv. 3 Key Sphere
The next portions of the game involve approaching Sin on foot after the team leaves the Airship.
The Airship will land and Tidus and company will move through the water towards Sin. Save your game at the Save Sphere and follow the rest of the team.
Note that at this point in the game you can still turn back around and look to complete some of the side quests rather than finishing the game. Going any further will head towards the final chapters of the game. This guide will provide ample warning before you reach the point of no return.
The mini-map for this area will contain some fog of war that makes it difficult to know which is the correct path.
Use the grey looking water walkway to find your way through this location. Just follow the red arrow north as you work your way through the Sea and use the map below (as a generic guideline) to help you get through.
There are treasure chests throughout the area which contain the following:
(1) 
Phantom Ring
(2) Special Sphere
(3) Elixir
(4) 
Wizard Lance
(5) Lv. 3 Key Sphere
Note that there are waterfalls along each of the pathways though so in many cases you will have to take the long way around in order to reach the treasure chests shown on the map. Because of this, you should make your way to treasure chest (1) first, followed by chest (2) at the top of the waterfall.
The Behemoth Kings in this area (shown in the screenshot below) can be particularly difficult. They will unleash a Meteor counterattack once you have exhausted their HP which can cause up to 7000+ damage. It is highly recommended that you run away from them unless you have trained up using the side quests, obtained your Celestial Weapons, etc.
Some alternative strategies for surviving Meteor, if you want to brave it, is to cast Protect on your party members and have them use the Defend command when Behemoth King is getting low on health.
Also note that Meteor is not triggered if Behemoth King is killed by a counterattack which means that you can have a character with the Counterattack ability use the Provoke ability when Behemoth King’s HP is low and just wait for it to eventually kill itself.
The Behemoth Kings in this area (shown in the screenshot below) can be particularly difficult. They will unleash a Meteor counterattack once you have exhausted their HP which can cause up to 7000+ damage. It is highly recommended that you run away from them unless you have trained up using the side quests, obtained your Celestial Weapons, etc.
Save your game at the Save Sphere at the end of the Sea of Sorrows before you proceed on to the next area.
The upcoming boss fight against Seymour Omnis is an entirely magic-based fight. You should take some time to equip each of your characters with any armor that they have that protects against magic. This includes:
Fire Ward,
Lightning Ward,
Water Ward,
Ice WardFireproof,
Lightningproof,
Waterproof,
IceproofSOS NulBlaze,
SOS NulShock,
SOS NulTide,
SOS NulFrostMagic Def+Be advised that you may also want to remove any weapons that you might have equipped if they have the Firestrike, Lightningstrike, Waterstrike or Icestrike abilities. They could all potentially heal Seymour if used to attack him.
In Game Description: Casts spells once energized by the four Mortiphasms behind him. Their positions somehow affect his actions. Physical attacks turn the Mortiphasms to the left, while magic turns them to the right.
HP: 80,000
There are four floating discs behind Seymour Omnis that are called Mortiphasms. These disc will rotate throughout the battle and the colors which are closest to Seymour will determine the strength and the element of the spells he will cast.
The discs have four colors: orange/red for Fire, yellow/green for Lightning, blue for Water and purple for Ice.
Seymour will cast four spells every turn. If all four discs have a matching color he will cast the strongest versions of the spells (
Firaga, Thundaga, Waterga or Blizzaga) and he will also absorb that element if a spell of that type is cast on him.
He will also be weak to the opposite element. If three discs match in color he will absorb that element as well, however he will no longer be weak to the opposite element. Two matching discs makes him immune to the element while a single disc will make him resistant to that element. You can use the Scan ability to verify his elemental strengths and weaknesses.
Seymour will begin glowing red after he has been attacked six times. He will also begin casting Dispel at this stage of the battle and will begin using Ultima against your party. He stops glowing red after he has cast Ultima.
Seymour’s Ultima isn’t nearly as powerful as the one accessible in the Sphere Grid, but it can still do some significant damage (around 4,000 HP to each party member). Make sure you heal up before he casts it and use the Shield command if you are having trouble surviving it. You can also use Lulu’s Focus spell or Yuna’s Shell spell to increase your resistance to it.
This is one of the few fights against Seymour where you can make use of your Aeons. He will not banish them if you utilize them during the fight. You can also use Yuna’s Nul spells to reduce the amount of damage done.
Use Hastega on your party early on and use Armor Break and Mental Break. Just be aware that you may have to recast some of these spells if Seymour uses Dispel.
You can also attack the Mortiphasms to rotate them to reduce the likelihood of Seymour using his strongest attacks. Some of the Mortiphasms are in the back row so you will have to use either Wakka or Lulu to hit those ones.
Steal:
Shining Gem (common)
Supreme Gem (rare)
Drops:
Lv. 3 Key Sphere (common)
The Destination of Hatred
You will receive the The Destination of Hatred Trophy after you have defeated Seymour Omnis.
Go back to the Sea of Sorrow before you move forward and save your game at the Save Sphere for a free HP and MP refill.
The next portion of the game takes place “Inside Sin”.
