The One-Eye is a special enemy in Final Fantasy X created in the Species Conquest at Monster Arena. It requires at least four of each Eye-type fiends to have been captured, except for the Bat Eye, which is only encountered in Via Purifico, when capturing enemies is not possible. One-Eye uses the same model as the Bat Eye.
It is immune to four of five elements (Fire/Thunder/Water/Ice).
One-Eye has Auto Regen. Being a flying enemy, One-Eye can be difficult to hit with a normal attack, but its low Evade stat will work in the party's favor. One-Eye has a couple of attacks which can prove to be dangerous to an unprepared party. Shockwave will hit all characters for magic damage and inflicts various status effects: Confuse, Slow, Sleep (by 10 turns), Darkness (by 10 turns) and Silence (by 10 turns); and Black Stare is directed against one character for physical damage and causes Curse.
The simplest strategy at lower endgame levels is to use Sandy's Razzia twice (select "Fight!" and then "One more time!") or Anima's Oblivion once (in the HD, PAL, or International versions). Aeons are immune to the status inflictions and should survive the initial Shockwave, so the Magus Sisters should not have much trouble staying alive.
Another strategy is to use Auron's Banishing Blade Overdrive as first move. If Auron doesn't have his Celestial Weapon, or a weapon with the First Strike ability, having Tidus use Armor Break will also work. Wakka should use his Attack Reels Overdrive, landing all 2x to finish him off.
Alternatively, a well-geared Auron can kill One-Eye before it has the opportunity to attack. Equip Auron with a Auto-Haste armor and Masamune. Enter a battle with One-Eye and guard which each party member until it attacks and kills the party. Now that Auron has 1 hp, his damage will be much higher and should be able to kill One-Eye in subsequent attempts. A strength stat on Auron of about 100 is sufficient for this strategy.