At first its shell will be closed. After suffering 440,001 HP in damage, Catastrophe will come out of his shell and begin unleashing a variety of attacks as heavy damage. While in its shell it will use Skip turn and Toxic Cloud, which inflicts Poison, Confuse and Curse. While out of its shell it will use Toxic Spray, which inflicts Full Break; Demi, which reduces current HP of target party by 1/4; and Hundred Thorns. Toxic Spray will kill a party member unless they have either Break HP Limit or Protect, and its Hundred Thorns will do the same, except on the entire party.
Catastrophe 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 armor, 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 WarriorOverdrive 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 Catastrophe before it attacks, Auto-Life works as the backup.
An alternative approach is to use aeons since most of Catastophe's attacks are status-based and aeons are immune. Bahamut is well suited, particularly if he is upgraded with Haste and Protect along with a method of healing. He should use Shield before each of the Hundred Thorn attacks.