Triple AP Weapons
Depending on how much of the end game content you wish to explore at the end of Final Fantasy X, you may need to level up your characters, and the most effective way of doing this requires that you obtain Triple AP Weapons for each of them. This guide explains an easier method of obtaining Triple AP weapons other than through customization.
You will need Triple AP weapons in order to quickly level your characters using the Don Tonberry power leveling trick. Check out the Quick Leveling Guide for more information:
Obtaining Triple AP weapons is as easy as battling and defeating a One-Eye enemy as many times as you need to.
One-Eye (Species Conquest)
Triple AP weapons will drop from an enemy in the Monster Arena called a One-Eye (a bat-like creature). One-Eye is an unlockable monster creation which you can obtain by capturing all of the Eye-type fiends which include:
The battle against One-Eye is fairly straightforward, however it does have an attack that can inflict Confuse, Darkness, Silence, Sleep and Slow. You will need to make sure that one or more of your characters has armor with the Ribbon auto-ability attached to it.
Unfortunately there is a random aspect to the drops, so you may have to battle One-Eye multiple times. The weapons you are looking for include:
Horn of the Ronso
These weapons also sell for a lot of gil so this is a good method of farming gil as well.