In the first phase of the battle, Ouroboros will use various attacks from job classes, such as the Ninja's Shippûjinrai, the Ranger's Multiburst, and the Arcanist's Corpse. The phase ends once Ouroboros reaches 1/6 of his health.
During the second phase, Ouroboros can use Zero Dimension, an attack which instantly KOs one party member. A Safety Ring can protect a character from this attack. Additionally, at the end of each turn during this phase, Ouroboros uses Total Regeneration, which instantly replenishes all of his HP. To move on to the next phase, Ouroboros must lose 50,000 of his HP in a single turn.
During the third phase, Ouroboros still uses Zero Dimension, and adds a new attack to his moveset. Demonic Touch lowers all party members' BP by 1. This attack is a physical one, so use Utsusumi to evade it. The next phase begins once Ouroboros loses half of his health.
The fourth phase introduces Celestial Touch, an attack which increases Ouroboros's Physical and Magic Attack by 50%. Use a Dispel or a debuffing attack to neutralize these stat buffs. However, when Ouroboros loses half of his HP, he uses Armageddon, which not only deals heavy non-elemental damage to the party, but replenishes all of Ouroboros's HP. Survive Armageddon three times to move on to the final phase.
Ouroboros introduces two more attacks during the final phase. His first attack, Disaster, is basically the same as Armageddon—only that it doesn't replenish Ouroboro's HP. The second move, Divergence, disables all commands except Default and Summon Friend for a party member. There can be a maximum of two party members under the effects of Divergence at a time. The phase ends once Ouroboros's health bar is fully depleted.