Final Fantasy X

Well if ur stats are high and u have the Celestial weapons, then u can kill him in like a few seconds. Otherwise, go get the Celestial weapons and train inside sin.

If all youa re after is a Triple AP Weapon, just go get 50 Wings to Discovery. You can get them in these places (Taken from the item database FAQ)

+ Wings to Discovery
1. Mix Item
2. Customization:
Triple AP: 50 Wings to Discovery
Break HP Limit: 30 Wings to Discovery

Buy: N/A
Sell: 7,500 Gil
Steal: N/A
Drop: Shinryu - 1,2; , = Monster Arena
- Capture 2 of each: Splasher, Achelous, Maelspike
Bribe: Marlboro - 4 ( 540,000 Gil) = Calm Lands (135,000)
Great Marlboro - 8 (1,280,000 Gil) = Inside Sin/Omega Ruins (160,000)
Found: N/A
Other: Prize - 30 = Remiem Temple - Chocobo Race, Win with 3 Chests & No Pole

The best way to get them is to race at Remiem Temple (if you don't know how to get there, look in a FAQ) and do the chocobo race to get 30. Then, if you have plenty of gil, bribe regular Malboro's for the rest. If not, do this...

1.Capture 2 of each of these:
Splasher- Mt. Gagazet (on the mountain or in the cave)
Achelous- Mt. Gagazet (on the mountain or in the cave)
Maelspike- Mt. Gagazet (on the mountain or in the cave)

2. Talk to the man at the monster arena to unlock Shinyru.

3. Continually fight and kill/ overkill until you have enough of them.

4. Customise your weapon.

(If you acctually want to kill One Eye, I suggest training in Sin/Omega Ruins and collecting/powering up the Celestial Weapons until you are powerful enough)

Easiest way to beat One Eye is with the Magus Sisters. Sandy tends to hit One Eye for 99,999 each time and two shots of that kills him. Sometimes One Eye gets lucky with the randomness of the Attack patterns from the sisters. IE Sandy casting reflect. Also, One Eye doesn't always leave The Triple AP weapons. Sometimes its just mage Armor. The good thing about the armor though is that you can easily replenish your monetary funds and your still receiving a boat load of Magic Defense Spheres.
If you don't have the Magus Sisters, follow the advice of others. If the Stats of the Magus Sisters aren't anything near able to easily kill this guy, spend some time in the Omega Dungeon beefing up Yuna.

I had this same problem (yesterday actually). I tried 1st strike/aeon strategy, and One Eye would still prove too strong. Here's what I did:

* Get x99 Door to Tommorow and x20 Megalixir as prizes from capturing monsters (if you haven't already)
* Get a Rod for Yuna with 2 Free Slots. Customize Double AP (x20 Megalixir) and Overdrive -> AP on it (x10 Doors). This will be your temporary AP gaining weapon ... for now.
* Do the Don Tonberry trick. Equip Yuna with the weapon, set her overdrive mode to Comrade, then let her allies attack Don to boost her AP. Revive them when they are killed. Flee when Don stops using Karma.
* Watch Yuna's sphere levels go up 10-23 lv.s. Power her up.
* Rinse and repeat about 5 or so times.
* Challenge One Eye again, who should be a pushover now xD

If you summon and it's still hard, just try lvling first. Make sure you get Celestial Weapon. so your aeon can break damage. Just use Anima and use Pain twice.
That's all.

Give yuna first strike weapon, summon bahamut and use mega flare straight away. If you have some decent leveling, Bahamut mega flare should have no problem dealing 99999 damage. If by chance bahamut dies after the overdrive, have a multi-hit overdrive like wakka attack reels or tidus blitz ace to finish up.

What the others have suggested should help, however I have one thing to add which helped me a ton. If you dont know already, your aeons stats are based both on how high Yuna's stats are, AND how many enemies you've killed. I had hardly leveled Yuna at all when I was trying this, so my aeons were doing very little other than just absorbing that first hit. However, if you have a turbo controller, all you have to do is equip someone with a first strike weapon, talk to the arena guy, choose fight monsters, besaid, dingo. Then just set X to turbo, and use tape or a rubber band to keep it held down. Your party will continuously fight dingo's, one shotting them each time with the person with the first strike weapon (they have like 100ish hp...). If you do this for a while (overnight works great), your aeons will be as maxed as they can get without Yuna's help, and this should be plenty to make the ones with br dmg limit hit for 99999. Also, this works great for getting people to 999 kills so that Don Tonberry hits you for 999999 for the AP trick. You can do it with whichever character has the lowest max hp, and using that you can make the don tonberry trick give you 99 Slvls extremely easily (dont even need triple OD, just need triple ap/overdrive->AP).

right u want the Triple AP Fast and u dont want 2 waste 2 much time about it? some people say use this and that like Use tha Magus Sisters that do 99,999 Damage Max but if u want 2 do it faster juts toss in Yojimbo and get him 2 do Zamnato it cuts them in half instant death yeah it will decrease ur chances of getting it but theres always a chance so it can be very fast 2 u or very slow (a bunch of people also say that the LONGER the Battle Lasts the better chance of items u have so it means dont use Yojimbo just go non stop Attacking or if u have ur strongest characters with Quick Hit have one of them Cast Hastega (makes ur partys turns come faster) and mix it with a Quick Hit (a Normal Hit but just makes ur Turns come faster also) it will last longer than a Zamnato and still give u a higher chance of getting the items

I love it when people leave the original subject...
To kill that bastard at low lvls you HAVE TO use A LOT of summons...
If you're really weak, first, go back to the calm lands and build up all aeons overdrive.
Come back, start the fight and skip turns until yuna comes up...
and start summoning.

Your fight should be somewhat like this.
-Summon Bahamut
-Use overdrive
kills you in 1-2 turns
summon anima
-ATTACK or PAIN
enemy attacks you
-Use overdrive
enemy kills you
summon magus sisters
-use overdrive
enemy attacks you
NOT DEAD?
DISMISS
regular attacks from the magus sisters are too random, if one uses protect or auto life or decides to lay down and scratch their... wings... you'll waste time and regen will give you a hard time... so dismiss aeon.

If he is still not dead
your last hope is yojimbo

Pay him 1001 and hope for one eye to die in 2 turns

IF the flying bastard is still not dead, you need to train

but train yuna only, raising her stats raises aeons as well, 1-2 hours should be enough to kill him using only anima and the magus sisters

Hope that helped more than the presidential debate you received lol.

The absolute easiest way to kill One-Eye is with Aeons. It's much easier than customizing several confuse proof such armors for everyone. The trick however is getting Yuna's level high enough, otherwise, aeon gets 1 hit KO'd by black stare. On my 1st try, Yuna had completed her part of the grid, yet Anima took 9000+ damage from Black Stare.

Two ways to get Yuna's level up. Find a save point Inside Sin (or Omega Ruins) and just grind. Or make a Double AP, Overdrive -> AP weapon for Yuna and use the Don Tonberry trick. Capturing monsters from many areas should give you the Megalixir and Door to Tommorow you need to make such a weapon.

Once you do get sphere levels for Yuna, focus on a part of the sphere grid that raises Defence (against Black Stare) and Strength. Then One-Eye should go down in 2-3 easy hits.

I suppose most of the answers work, but my main questions are:

How far are you in the game?
Do you have celestial weapons unlocked?
How far are you characters on Sphere Levels? (IE, still in theirs, traveled into So&So's)
How many Aeons do you have AND how high are they?

I ask because, all this matters. If you're not that far your success is going to very limited if not impossible. I was able to defeat One Eye with - Wakka, Auron, and either Tidus or Kimiarhi in... without using summons and sometimes it only took 1 hit from each, sometimes it took 2 hits from each, but I took the time to get the correct armor and weapon customizing done to allow me to get first hits and auto-phoenix, I also had celestial weapons unlocked, and I had them all either starting their third or ending their third sphere grid person. I have also beaten One Eye using only the Magus Sisters, but there again, I still had to make sure I had the proper weapon and armor, sphere grid levels, and celestial weapons on to allow me to bring in Yuna for the summon. Which didn't always end well, so I didn't pursue this way very often personally.

There is no quick way to do it, it is going to take, energy, money and a lot of patience. So if you're a very 'anal' player, it should be very easy, if not, it will be your worst nightmare. I am a very anal player so it may have taken me awhile to get strong enough to do it quickly, but still the Triple AP sure did make life a lot easier after that, (and the bad ones sold for a good amount too!)