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'sRazzia twice (select "Fight!" and then "One more time!") or Anima'sOblivion 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.
Alternatively, a well-geared Auron can kill One-Eye before it has the opportunity to attack: Auron with a Auto-Haste armor and Masamune. The player should guard which each party member until One-Eye attacks and kills the party. Now that Auron has 1 HP he will deal maximum damage with his Masamune, and should be able to kill One-Eye. A Strength stat of about 100 is sufficient.