Alright. I know. The bosses I mention in the title are hard. Especially Yunalesca. But don't worry.
Fight with: Yunalesca
HP: 24000
Place: Zanarkand Ruins
Preparations:
Have everyone's Overdrive gauge full, especially Yuna for Grand Summoning. Stock lots of Holy Water, Mega Phoenix or Phoenix Downs, Echo Screens and Eye Drops. Esuna or Remedies should do it, as well. Other way is to customize Lulu's armor and add 'Silenceproof' ability because she will certainly be your greatest asset in this fight. Get yourself a handy amount of Ether you can get by stealing Ethers from Behemoth that exists along the way to the Dome. 10 Ethers should be enough but stock more in case you are stuck. The reason I told you to get Ethers will be explained below. Equip Yuna with 'Magistral Rod' that had 'Half MP Cost' property because she'll be hell a lot of healer. Once you're ready, enter the dome and the battle will begin.
Battle:
First, had Rikku stole a Stamina Tablet from Yunalesca, and then keep pounding her with your best physical fighter and mages. The things you might find annoying are when Yunalesca attacked with physical attack she will counter with Blind and when attacked with magics she will counter with Silence. That's why I said stock lots of Remedies, Echo Screens and Eye Drops. Don't hesitate to use Ether on your character if she ever drops Osmose in the middle of battle [and her Osmose drains a hell of lot of your MP]. With only 24k HP, she's not that tough for a boss battle...
Fight with: Yunalesca
HP: 48000
Place: Zanarkand Ruins
To begin the battle, Rikku must steal Stamina Tablet and Mix a Mega Vitality to double everyone's Max HP. Quickly heal everyone because once it's Yunalesca's turn, she will use the pain in the ass Hellbiter that causes Zombie and Curse. Don't summon your Aeons yet, Yuna must keep pushing the party with her Curaga and Regen spells. The nice thing you might find at this battle that you are free to attack Yunalesca because she won't counter any attacks, but her Mind Blast probably is the worst attack she can have. To kill your party, Yunalesca had the awful equation of Hellbiter + (Regen * Curaga) = Death. Curaga will instantly kill your character since it will deal around 5000 damage but Regen is another story. If any of your character was inflicted by Regen, quickly dispose their Zombie status so Yunalesca won't damage your party, instead, she's healing you for free! But be aware and keep at least a Zombied character in your party because...Well, you'll see below...
Fight with: Yunalesca
HP: 60000
Place: Zanarkand Ruins
In her final form, Yunalesca loves to drop Mega Death at the beginning. Mega Death INSTANTLY KILLS any characters that is not Zombied, so like I said, keep at least a Zombied character along the battle because she loves to drop Mega Death in the middle of battle as well. Play defensively in this battle but keep bringing her down with Tidus and Lulu. Should Lulu or Tidus runs out of MP, use Ether to replenish their MP. Yuna must keep pushing the party with supportive spells and if you had any chance, Grand Summon your strongest Aeon [in my case, Bahamut] and pound Yunalesca with your Aeon's Overdrive, should they have their Overdrive gauge full and with Grand Summoning, you will have another full Overdrive gauge and since Aeons' Overdrives always do 9999 damage, they can bring this final form down. Don't die here, play defensively but don't forget to bring Yunalesca down, and stay alive.
Fight with: Left & Right Fin
HP: 65000 each
Place: Skies of Spira
This boss' only offensive spell is Gravija and you can easily avoid this spell by ordering Cid to move away from the fin. 65k HP is not that tough, just have Auron use Armor Break on the fin, Lulu casting magic from far away and Yuna using Aeon's attacks. With Armor Break, you will find the damage Tidus deal will skyrocket, and under the influence of Hastega you should be able to bring these things down even before that guy had a turn. Because Sin will mostly remains motionless, this is a very easy boss battle compared to Yunalesca.
Fight with: Sinspawn Genias
HP: 20000
Fight with: Sin
HP: 36000
Place: Skies of Spira
The same strategy for the fins applies, it just that this guy had Sinspawn Genias that is considerably awful if you don't take care of it soon. With only 20k HP, this guy shouldn't be too much of a brawl but then once it hides in its shell, kill this guy with magic. After you kill Genias, you can face Sin. Have Auron use Armor Break and Tidus casting Hastega. 36k HP shouldn't be too much damage and since Tidus' damage is skyrocketing, you might find yourself laughed so hard because Tidus kills this boss with only less than 15 turns.
NEXT ON: OVERDRIVE SIN
Stay tuned! :p
~Let me be your illuminating shield...~