The Earth Eater is a fiend in Final Fantasy X bred as part of the Original Species of monsters in the Monster Arena. It resembles the Chocobo Eater fought at the Mi'ihen Highroad earlier, but is much tougher. To unlock the Earth Eater, at least two Area Conquest monsters have to be bred, though not necessarily defeated.
Earth Eater has Auto-Reflect and can instantly kill characters with its Megaton Punch attack/counterattack, which it will use twice consecutively on its turn if a party member survives the first attack. Like the Chocobo Eater, Earth Eater can be knocked onto its back by dealing over 100,000 damage to it. It will still counterattack while on its back, but switches to reflecting Flare off itself and won't act for its next two turns before standing back up. Like all Original Creation monsters, the 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 hidden 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 with low life (9999 and below) and the Entrust ability. The party should equip their Celestial Weapons (if available) as even piercing weapons inflict minor damage compared to fully powered Celestial Weapons. Wakka should have high Strength and Agility, preferably close to maximum.
A fast strategy is to expend all of the party members' 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 life possible to die instantly from Flares to fill the Overdrive gauge.
The fastest strategy is to use the Loner Overdrive Mode with Wakka and simply knock down the Earth Eater with two hits of 99999 damage, then have the other two characters escape (or KO). All party members must be equipped with Auto-Haste armor and Wakka with Deathproof armor, then keep pressing until the Overdrive gauge is full and attack with 12-hit Attack Reels, then repeat to defeat it. The sequence must be completed quickly to inflict maximum damage.
Shell can be used on Wakka to protect from the Flare counterattacks if his Magic Defense isn't high enough. Tidus's Slice & Dice can also be used if Wakka's stats aren't high enough. When Earth Eater stands up before the player finishes him off, casting Auto-Life protects against his instant kill counter.
Another strategy for lower levels is to set Yuna's Overdrive Mode to Avenger and give the party Auto-Phoenix. The party should hit Earth Eater a few times to deal damage. Yuna's Overdrive charges by having her party members die to Earth Eater's counterattacks—equipping Nirvana helps. Valefor's Energy Blast or Anima's Oblivion can be used to dish out the 100,000 damage needed to knock Earth Eater onto its back. The aeon Overdrive turn penalty will balance out with the turns needed for the Earth Eater to climb back up, giving the player one turn to dismiss the aeon. Repeating this twelve more times wins the battle. Auto-Haste helps, but is not strictly necessary.