Before the battle, the party should be equipped with the Holy Shields, as Chaos uses powerful Light-elemental spells. The Ribbon is also a necessity, as are the legendary artifacts. If you have the Hermes Sandals, give them to the character equipped with Lux. It is also advisable to equip all characters, regardless of job, with a healing spell, as there will be times where everyone needs to concentrate on it. During the encounter the player should bring an Elementalist, a healer, and two damage dealers. A Black Mage is especially useful for the Magic Might and Desolator spells. The Bard's Ditty, Motet, and Aria abilities can also be very useful. Either a White Mage or a Salve-Maker with a good item stock can serve in the healer role.
First Form Edit
The first form of Chaos uses a variety of White and Black Magic against the party and acts twice per turn. During the entire battle Chaos will cast Darkra, Darkaga, Banishra, and Banishga on the party. Chaos constantly casts Protect, Shell, and Haste on himself. During the later stages of the battle, Chaos begins to cast Slow and Stop on the party.
As usual, the player should immediately cast Lux and Mysterio, and after that concentrate on healing and dealing damage. The Black Mage's Mirror ability is also useful, particularly if the player is using a Salve-Maker (their healing won't be accidentally reflected while Mirror is up).
The first form of Chaos will fall to steady attacks. However, when the music changes to reflect Chaos' critical health, it's advisable to switch to normal attacks to conserve AP for the second form, as there is no chance for restoration between them. It's especially important that the character with Lux finish the battle with enough AP to immediately cast it at the start of the next.
Second Form Edit
Like his first form, the second form of Chaos acts two times per turn. During the beginning of the battle Chaos uses only normal physical attacks and the Fly Swat ability, which hits the entire party. After his HP falls below seventy-five percent, Chaos begins to use Blaze, Earthquake, Tsunami, and Lightning. These abilities deal elemental damage and cause certain status ailments on the party. They can also cause Instant Death.
Like the previous battle, the player should immediately cast Lux and have their Elementalist use Mysterio; keeping Mysterio active is especially important here because Chaos uses all of the elements in his attacks. The healer should heal every turn (the Miracle spell is particularly useful for full healing and removing status effects, but a Salve-Maker with a Remedy is also useful). Damage-dealers should use their most powerful abilities whenever they have the AP for it unless they need to switch to healing.
When the second form of Chaos is defeated, the entire party is fully healed.
Third Form Edit
Chaos in his third form is joined by his two arms in battle. The third form of Chaos acts twice per turn, usually once for each arm. The Left and Right Arm can only be attacked in the beginning and Chaos cannot be attacked until his arms are defeated. Chaos usually opens the battle by using Black Hole, to teleport a party member from the battle for four turns. His Right Arm uses the physical attack Hurl, which throws one character into a second one to damage both, and his Left Arm has a variety of magic spells in its arsenal. After both arms are defeated, Chaos enters the battle and begins to use the attacks he used in his second form and Big Bang to attack the entire party.
When battle starts, the player should use Lux and Mysterio on the party. The player should use techniques that can attack both arms at once, such as Bladeblitz or Magic Might. The player should repeat this strategy for a couple of more turns until both of his arms have been defeated and keep their support and healer characters healing, and keep Mysterio up.
After entering the battle Chaos will start to cast Big Bang on the party, which can deal heavy damage. Concentrate on healing with Healthcare + X-Potion or Curaga. If the player's healer has been absorbed by Black Hole, the best way to recover from this attack is to use X-Potions or cast Cura on other party members. After he uses Big Bang, the player should have their damage-dealers concentrate on their strongest techniques.
During the final stages of the battle Chaos will start to use Big Bang twice in one turn, which can deal heavy damage on the party and can even kill the player's White Mage and Black Mage. If this happens the player should use a Phoenix Down or Raise to revive KO'd allies. After the player is done fully healing the party, the player should have their Hero and Black Mage attack Chaos, White Mage healing the party, and having their Elementalist recasting Mysterio. Once he is defeated, Chaos will cast Bigger Bang and the battle will end.