Final Fantasy X Tips

Get a 3 or 4 slotted weapon and customize Triple AP, Overdrive -> AP, and Triple Overdrive (you can't customize triple AP with double AP and triple overdrive with SOS Overdrive or Double Overdrive) and something else if you want. Now, get an armor with at least one slot available and customize auto-pheonix (which you need 20 mega-pheonix for if I remember correctly) on at least 2 characters, and a lot of Pheonix Downs!! Now unlock Cactuar King in the Monster Arena (you need to capture at least 3-5 monsters in the Thunder Plains IIRC) and fight him. Oh, and you need to set everyone's overdrive mode on STOTIC.

Now, get the characters you want leveled up in the fight, and have one character attack the Cactuar King. He should counter with 10,000 Needles, which hits for 10,000 damage, and with Overdrive to AP and Triple Overdrive, that's already 30,000 AP right there! Plus Triple AP which would be 90,000 AP!

He randomly attacks a character, so you should put all of them on Stotic or all Comrade.

Oaka the Merchant Extraodinaire's brother sells weapons with 4 blank slots. If you need Gil, fight a Mimic (found in Omega Ruins) with Gillionaire equipped to somebody when you win the battle, you get 10,000 Gil plus a bit from the other monster, or if you've already unlocked One-Eye, battle him and he drops weapons with Triple AP, and you can sell them for a lot of Gil!