As a Mortiphasm disc rotates, a colored section will face the side closest to Seymour; these will determine which spells Seymour Omnis will cast at the party each turn. The battle will start with the four Mortiphasms as Fire, making Seymour cast four Firagas unless the player changes them. One of the first actions should be to forcibly rotate one by attacking it, this will prevent a barrage of powerful spells being cast one after the other.
Physically attacking a Mortiphasm will cause it to rotate to the left, and using magic will cause it to rotate to the right. However, the Mortiphasms, unlike Seymour Omnis, are out of reach, so only Wakka can attack them physically. If there are one or two of the same colors pointing at Seymour, he will cast a second tier spell (Fira, Thundara, Blizzara, Watera).
If each color is the same, the spells will have increased power. If they are different, the spells will have less power. Seymour Omnis's magical weakness is determined by whichever side is furthest away from him when he is attacking. After each time he casts Ultima, the discs will rotate so a new element is chosen. These foes are immune to damage, so the party should focus their attacks on Seymour.
While it is normally impossible kill the Mortiphasms, if the player is to modify the game to allow abilities that pierce immunity, Mortiphasm's demise freezes the game.