FFXII - Walkthrough - Sky Fortress Bahamut
You'll start off in the Antechamber, with two sets of doors: you can go either way. After going through the doors and venturing forward a bit, you'll notice a set of stairs to your side (left or right, it depends) leading up - follow these stairs and proceed to the next area.
Once there, a cutscene, and then back to gameplay: some more enemies here, but be careful to not let too many gang up on you as it can hurt - numerous status ailments and all.
Go to the next area: an elevator. Do any preparations you need to before touching the Lift Controls... once you do, you'll get a cutscene, and a boss battle vs. Gabranth (again)
Gabranth
Status: Haste
Steal: Hi-Potion
Attacks: Regular (~550, comboable), Sentence (~1000 single), Renew (total HP recovery), Innocence (~1350 range), Enrage
Advice: After you take off a chunk of his HP (around 1/4 to 1/3?) he'll say a line and use Renew... completely healing him, so it's a brand new ballgame. Gets the defence boost at HP Critical. If you have trouble with this fight, you shouldn't be here, as the final battles are up ahead... so hopefully you don't have trouble!
Once it's done, you're back on the elevator. Heal yourself, and buff yourself with whatever statuses you want, because once you touch the Lift Controls... a cutscene, and the final battle sequence begins!
Vayne
Status: None (Haste after dialogue)
Steal: Nothing
Attacks: Kick (~730), Lunge (~700), Regular (~650, comboable), Pummel (~1000 single), Mach Wave (~800 range), Force of Will (~1200 all)
Advice: Partway through, busts out his first major move (Mach Wave) which attacks in a straight line, so its range isn't too worrisome. At some point he'll suddenly gain Haste, so be sure to dispel it. At HP Critical, busts out Force of Will (which, if it doesn't hit everyone, certainly has a wide range). But in the end, this is the first of the final battles, so you shouldn't expect much in terms of difficulty anyways!
Afterwards, another cutscene, and another boss fight!
Vayne Novus
Status: Protect Shell Haste
Steal: Nothing
Attacks: Blizzaga (~1000), Aeroga (~800), Azure Vortice (?), Contempt (~900 single), Crimson Vortice (?), Inviolable Will (~950 all), Magick Barrier, Thundaga (?), Tree of Sephira (~1900 all), Limit Break (?), Firaga (~1100), Regular (~500 comboable), Waterga (~800)
Advice: This time, Larsa is passed out and Gabranth assists you! Though the battle, numerous Sephiras (Vayne's flying swords) encircle the field, occasionally attacking you. I wouldn't advise paying any attention to them - your gambits should be adjusted so all your focus is on Vayne Novus. The Sephiras will hit for about 200 damage and cast the occasional status... if you want to attack them, be my guest, but I always considered them to be more of a nuisance than anything.
A "Vortice" is followed up by one of his major, flashy animated attacks. Tree of Sephira is certainly the most daunting, though his Magick Shield wore off immediately after performing it. For this reason, you might want to destroy the Sephiras - but in the time it would take to lock onto and destroy 5 rapidly-moving objects, all those hits could be inflicted on Vayne Novus. So I still recommend focusing your attacks on him, and dealing with his major moves on you as they come. And they will come - but so long as you're an appropriate level, with appropriate equipment and respectable base HP (at least 4,000), you should be adequately prepared for this battle, and specifically, the final battle ahead.
Once it's done - another lengthy cutscene or two, and then the big moment... the final boss battle!
The Undying
Status: Haste
Steal: Dark Matter
Attacks: Regular (~1500), Piercing Thundaga, Blizzaga, Firaga, Holy (~1500-2000), Divine Sword (~2000 single), Megaflare (~1300 all), Chain Magick (Zero magick charge time), Faith, Magick Shield, Ascension (~2400 all), Enrage (Zero attack charge time), Bravery, Force Barrier (Paling), Gigaflare Sword (~3800 all), Perfect Defense (Paling + Magick Shield), Piercing Dispelga, Teraflare (~3800 all)
Advice: The final battle... can you do it? The Undying appears to have four different "modes".
Intro: the way he is at the start of the battle, before ever casting Paling or Magick Shield. Sticks with his "regular" attack and Piercing -aga spells, all which do around 1500-2000 damage. Major attacks include Divine Sword and Megaflare, but their damage incurred is minimal nonetheless.
Magic mode: In quick succession, he'll cast Chain Magick (eliminating any wait time for magick), Faith (to increase magick power), and Magick Shield. At which point, it'll be impossible to dispel Faith, so you'll be stuck taking extra damage from whatever magicks he casts. He'll finish off this "mode" with Ascension, and the Magick Shield will dissapear afterwards.
Attack mode: In quick succession, he'll cast Enrage (eliminating any wait time for attacks, I suppose?), Bravery, and Force Barrier, which erects a Paling around him. His physical attacks will do ~2200 with Bravery, so I strongly recommend dispeling it a.s.a.p., as with Enrage up they'll be coming frequently and it could put you in a bit of a pickle. Protectga and Bubble are a wise combination, as this "mode"'s finisher, Gigaflare Sword, hits everyone for tremendous damage, far more than Ascension.
Perfect mode: Perfect, you say? About as perfect as The Undying gets. At HP Critical he pulls out all the stops - first, he gets the assorted boosts most bosses get at HP Critical; defense up, magick up, strength up. Then he uses Perfect Defense, rendering him untouchable. He might play around with a few attacks or spells, until it's finisher time - he'll use Piercing Dispelga to ensure you have no protections, and then bust out Teraflare, another crushing attack. Afterwards, his Paling and Magick Shield should both collapse.
In conclusion, what to expect:
-Chain Magick -> Magick mode -> protect against magicks with Shellga
-Enrage -> Attack mode -> protect against physical attacks with Protectga
-Perfect Defence -> He's going to hurt you -> make sure your HP is topped up (at least over 4000, preferably higher), and it might be a good time to equip Bubble Belt if you haven't yet
-He doesn't appear to use any attacks which inflict statuses, so don't worry about status defence gambits & equipment
-Enrage and Chain Magick only seem to be used once, but once tends to be enough
So now you know about The Undying. His assorted attacks and spells, and his defensive tendencies. But what can you do to improve your chances in battle?
(1) First off, don't forget that this is the final battle - there's no New Game + and no more save points. So use your items! Too often I find I hoard my elixirs and megalixirs throughout the game, only to never use them whatsoever. If you got megalixirs, no better time than the present to use them.
(2) Choose your accessories wisely. Your Golden Amulets and Embroidered Tippets are useless now: Bubble Belts are a good idea for the extra HP Padding, Sage Ring is ideal for your primary magic wielder, and Genji Gloves could be a good fit for your primary offensive unit.
(3) You don't need to worry about surprises; he's not going to summon allies, and he's not going to Renew/Restore any of his HP. Other than his occasional physical/magickal immunities, it's a straightforward battle, so just do what you've always done: hit & heal. There's no HP bar so you can't keep track of your progress until he hits HP critical - then you'll know you're near the end.
(4) While Piercing Dispelga will be a pain, don't be afraid to be liberal with your buffs. Shellga, Protectga, Hastega will certainly help you out! And speaking of Dispel, it should be your very first move of the battle (to remove Haste), and should be cast whenever he uses Bravery or Faith on himself (so long as you're able to - don't forget about Magick Shield)
(5) To repeat point (1): if your MP is low, don't use Charge - go with a Hi-Ether or Elixir. It would surely be a pain to reach a critical juncture of the battle, and use Charge, only to have your MP reduced to 0, not only wasting a turn, but possibly wasting a turn at a very bad moment!
(6) My personal recommendation - once his Paling falls after Teraflare, use a Megalixir, and then go right into a Quickening chain. Your party will be at full MP and he'll be at HP Critical - it will give you a solid shot at finishing him off. But again, only do it if the Paling isn't up, otherwise it will all be for naught.
After you defeat The Undying... that's it! Enjoy the final cutscenes, and look forward to the eventual sequel, Final Fantasy XII: Revenant Wings!
I hope my walkthrough helped you out in some way - but now that you've completed the game, you might want to go back and do other things you skipped, such as mark hunting, acquiring espers, or completing the bestiary. If so, FF:WA's FFXII section is loaded with guides to help you get the most out of your game. And if you can't find a solution to your query, feel free to visit the Final Fantasy XII forum, ask your question(s) there, and one or more of our hundreds of active members will be sure to help you out!