nononononono

You have to get the three eyeball specie monsters.

There's one in macalania, one in the omega ruins, and another in Zanarkand, or the road leading up it.

Go to those places and capture them using the weapons you buy at the monster arena in the calm lands, and when you capture a certain number, go back to the calm lands, and you'll have unlocked One Eye. This is a species boss, so when you're weak, he'll be pretty hard.

When you fight him, he'll drop the triple AP weapons, which means you gain sphere levels THREE TIMES as fast. If you put overdrive-> AP and Triple Overdrive, you gain levels very quickly.

Nothing to do with legendary or celestial weapons