Catastrophe is a special monster in Final Fantasy X bred as part of the Original Creation in the Monster Arena, and becomes available after six monsters have been bred in the Area Conquest section. Catastrophe looks much the same as Sinspawn Geneaux fought on the steps of Kilika Temple, and Sinspawn Genais that is fought on Sin.
When the fight begins, its shell will be enclosed, periodically releasing Poison. After suffering 500,000 HP in damage, Catastrophe will come out of his shell and begin unleashing a variety of attacks which inflict all Breaks and Demi, as well as heavy damage.
While in its shell it will use Skip turn and Toxic Cloud. While out of its shell it will use Toxic Spray, Demi, and Hundred Thorns. Toxic Spray will kill one character unless the player has either Break HP Limit or Protect, and its Hundred Thorns will do the same, except on the entire party.
It drops weapons with status strikes and armor with status proofs. Its equipment drops provide a 6% critical hit chance bonus instead of the usual 3% (this is relevant to weapons and armors, but only to attacks that deal physical damage).
A good strategy is to have have a party with Yuna, Wakka, and another person of player's choice. The player should have all of their Celestial Weapons. Wakka should have assigned Warrior Overdrive Mode. The first recommended ability to be used is Hastega with whoever goes first. The player will have plenty of time to buff up, as Catastrophe does not attack or counterattack until later. Yuna should cast Auto-Life on all party members while Wakka and the other character begin attacking.
Whenever Wakka gets his Overdrive gauge full, he should use Attack Reels and attempt to land all "2 Hit" blocks, making for 12 attacks total. When Catastrophe comes out of its shell, other characters' Overdrives can be used, as they deal much more damage now. If the player fails to kill the fiend before it attacks, there should be no problem as they player should have already cast Auto-Life on all their characters.