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Obtaining 99,999,999 gil is not as hard as you would think. It requires mastering "All" materia one after another which becomes increasingly easier the further you are in the game. There are two effective methods for doing this, one is grinding just outside of Mideel and the other is the Northern Crater, specifically the Lake/Water Crossing and Lake/Water Path screens.
You'll already have the Highwind at this point which means Fort Condor is selling All materia for 20,000 gil each. If you don't already have several to level up, get as many as you want/need to make this grinding process faster. I recommended filling all by 1 slot on each character, leaving room for a powerful magic, summon or Slash-all that you might need depending on how strong you are. If your characters are above level 60, you should be just fine using regular attacks around Mideel and even in the Northern Crater.
Mideel in my opinion is much faster simply because you have access to it earlier and by the time you've reached Mideel you should be strong enough to wipe the enemies with a single spell, summon or Slash-All.
Northern Crater is an amazing source of AP by feeding the Elixirs to the Magic Pots or defeating Movers (little red bouncing balls). The downside here is travelling into the crater and going all the way to the specified maps to grind. There are other encounters here that give a moderate amount of AP but ultimately slow you down. It will be up to you to choose from a steady medium amount of AP every single battle, at a fast pace or a very large amount of AP at a slower pace.
Magic Pot yields 8,000 EXP and 1,000 AP (2,000 or 3,000 with double/triple growth. You can encounter a lone Magic Pot (always alone) or two Magic Pots (never accompanies by other enemies) for 16,000 EXP and 2,000 AP (4,000 or 6,000 with double/triple growth).
Movers (a little over 4% chance encounter) always appear in groups of three and yield 800 AP each for a total of 2,400 AP per encounter (4,800 or 7,200 with double/triple growth).
All that is left to do is choose the best party for the job. I recommend Cloud and Cid because they both have the only Triple growth weapons in the game. Cloud's Apocalypse is 3x slots triple growth slots and Cid's Scimitar is 2x slots Triple growth which means fewer materia slots but maximum AP. Alternatively you can use Cid's Javelin, which coincidently has the most double growth slots for a weapon at 5x slots, it's up to you. Your third party member is irrelevant but I usually go with Yuffie equipped with her Rising Sunbecause her Conformer doesn't suffer from damage reduction penalty while using Morph and by this point in the game I am alternating between AP grind and Source morphing, so it's easier to keep her in the party to to prevent swapping characters all the time. Lastly, you'll want to equip Rune Armlets on everybody.
Cloud: Apocalypse (3x slots triple growth) or Rune Blade (4x slots double growth) and Rune Armlet (4x slots double growth).
Cid: Scimitar (2x slots triple growth) or Javelin (5x slots double growth) and Rune Armlet (4x slots double growth).
Yuffie: Rising Sun (4x slots double growth) and Rune Armlet (4x slots double growth).
Encounters around Mideel or head deep into the Northern Crate until you reach the branching paths. At the first branching path, go left and then right on the next screen to reach the first Water area and if you go right again, the second area. If you choose the Northern Crater, use one or both of these maps for Magic Pots and Movers.