The Earth Eater is a special fiend in Final Fantasy X bred as part of the Original Species of monsters in the Monster Arena. It physically resembles the Chocobo Eater fought at the Mi'ihen Highroad earlier in the game, but is much tougher. To unlock the Earth Eater, at least two Area Conquest monsters have to be bred, though not necessarily defeated.
It has Auto-Reflect. The Earth Eater can instantly kill characters with its Megaton Punch attack/counterattack, although characters equipped with Deathproof and/or Auto-Phoenix can help negate this. Like the Chocobo Eater, Earth Eater can be knocked onto its back when suffers over 100,000 damage. It will still counterattack, but switches to reflecting Flare off itself. Like all Original Creation monsters, Earth Eater has no trouble attacking characters regardless of their evasion.
Earth Eater will drop weapons with Triple Overdrive, armor with Auto-Potion, and a Fortune Sphere. Its equipment drops provide a 6% critical hit chance bonus instead of the usual 3% (this is relevant to weapons and armor, but only to attacks that deal physical damage).
When farming Earth Eater for Fortune Spheres, the player will want to kill it as fast as possible. The party should have two backup characters to have low life (9999 and below) and the Entrust ability. The party should equip their Celestial Weapons (if available) due to the fact that piercing weapons inflict minor damage compared to the Celestial Weapons at full power and preferably use the Avenger Overdrive Mode, and Auto-Phoenix is useful as well. Wakka should have high Strength and Agility, preferably close to maximum.
The fastest strategy is to expend all of Overdrives with 12-hit Attack Reels. Every instant party member death will fill two Overdrive gauges by ~95%. Characters should Entrust them to Wakka. To farm the Fortune Spheres it is best not to hit the save sphere between battles because the party should have the lowest amount of life possible to die instantly from Flares to fill the Overdrive gauge.
Another fast strategy is to use the Loner Overdrive Mode with Wakka and simply knock down the Earth Eater, then have the other two characters escape. One should cast Haste on Wakka if he does not have Auto-Haste armor equipped, then keep pressing until the Overdrive gauge is full. If this strategy is executed correctly, the player will be able to defeat Earth Eater before he can stand up again. Shell can be used on Wakka to protect from the Flare counter attacks if his Magic Defense isn't high enough. Tidus's Slice & Dice can also be used to perform this strategy if Wakka stats aren't high enough. In the event that Earth Eater stands up before the player finishes him off, casting Auto-Life protects against his instant kill counter.