Its shell deflects physical attacks well. Tentacles absorb magic attacks. Defeat tentacles first, then force it from its shell with fire magic. Absorbs water-based attacks.
Sinspawn Geneaux is the third Sinspawn encountered in Final Fantasy X, left behind after Sin attacks Kilika. It is assisted by two tentacles that sprout from the ground on either side of the player party.
The tentacles will intercept any spells directed at Geneaux, including group attack spells like Demi, and their attacks will inflict Poison. Geneaux will initially stay inside his shell and is protected against physical attacks. Once the tentacles are defeated, it is vulnerable to magic, particularly Fire-elemental spells. When its HP drops past 2,400, the shell will open and Geneaux will begin attacking by using Poison- and Water-based spells. At this stage it becomes susceptible to physical attacks.
To skip fighting the tentacles, it's best to have Valefor's Overdrive gauge filled and use Energy Ray to kill the tentacles and, if Valefor's strength is high enough, make Sinspawn Geneaux come out of its shell. If it doesn't come out, a couple of Fire spells should work. After Sinspawn Geneaux comes out of its shell, its Poison and Water spells can be countered by using Wakka's Silence Attack or Yuna's NulTide spell. Now that it can be hit physically, Tidus and Wakka should be able to finish the battle easily.