I'd made it out of the frying pan, and into the freezer.
—Tidus escaping Geosgaeno the first time
Geosgaeno is a geo-type fiend in Final Fantasy X that appears on the Baaj ruins to threaten Tidus when he tries to find shelter in a derelict temple near the beginning of the game. After three turns, Tidus wisely decides to flee. Later on, once the party has gained control of the Fahrenheit, the ruins can be revisited, and Tidus, now accompanied by Wakka and Rikku, can attempt to defeat the boss and enter the Chamber of the Fayth which houses the fayth of the hidden aeonAnima.
Geosgaeno can drop weapons with No Encounters, and armor with Auto-Reflect. Since No Encounters cannot be customized to a weapon, only on armor, defeating Geosgaeno is the only way to obtain a weapon with this ability with a 50% possibility of it dropping one.
When encountered in the beginning of the game, Geosgaeno's every attack halves Tidus's current HP. After three turns the battle will end with a cutscene. If the monster is killed (only possible through use of a cheating device, such as GameShark, to alter Tidus's stats), the cutscene will play as if the battle had ended normally. One should not waste turns and Potions healing, as every attack simply halves Tidus's HP.
In the later battle, Geosgaeno attacks with Stone Punch, which may cause Petrify. Because the fight takes place underwater, petrified party members will instantly shatter, making them inoperable. Geosgaeno uses KO Punch to always inflict Curse and it may instantly kill a character. Geosgaeno has 95% resistance to Poison.
Geosgaeno occasionally inhales a party member. When this happens, all positive status effects, such as Protect or Haste, will be transferred to Geosgaeno so long as the character is contained within it. The inhaled character may Struggle with a Trigger Command or defend. If the inhaled does not Struggle, on its next turn Geosgaeno will exhale the character, leaving them with one HP and dealing the inhaled's current HP minus 1 damage to each party member. If they do Struggle, Geosgaeno will exhale and only KO the one who was swallowed. If a character is swallowed and Geosgaeno is defeated, it will deal 9,999 points of damage to all party members.
One should never defeat Geosgaeno if a party member is swallowed. A simple strategy is equipping a weapon with any elemental affinity (e.g. Firestrike) onto Tidus and use Quick Hit or, if possible, customize a empty four slot Variable Steel with all four elemental strikes to change into Crystal Sword for Tidus for maximum damage, but it will cost 100,000 gil. Hastega is a good option if the party doesn't have Auto-Haste. Rikku can use elemental crystals and Wakka can use his OverdriveElement Reels.
Alternatively if Spare Change was picked up on Rikku and the player has the extra cash, Geosgaeno can be killed on one turn by tossing 10 times its health at it. Given it has 32,767 health that would be 327,670 gil. The cost can be reduced if the party has charged Overdrives they use before using Spare Change to lower its health. Using a Sensor weapon, the player can see how much health it has and throw 10 times that amount to kill it.
Another possible strategy is to equip either Tidus, Wakka or Rikku with the Evade & Counter armor. When a party member gets sucked in they must not struggle but defend by pressing . The character with Evade & Counter may also use Guard or Sentinel to nullify Geosgaeno's punch attacks.