(Ahriman? That one was different from the one we've beaten before...)Kain Highwind
Because Ahriman opens the battle with a Doom spell that hit the entire party, time is of the essence. Be sure to bring plenty of Phoenix Downs to this fight. A good strategy when fighting Ahriman revolves around Kain. If Kain has a lance equipped, he will do extra damage to Ahriman because it is a flying enemy. This being the case, it is recommended to have Kain use Jump exclusively when he isn't using Phoenix Downs to revive fallen party members. To augment the damage of Jump, it is recommended to fight Ahriman on a New Moon. Because White Magic is virtually useless in this battle, the penalty during the New Moon phase is negligible. It's recommended to only revive fallen characters after the Doom spell has been cast once again on any party members without the ailment. Bands are also not recommended, because they take a long time to be performed. That being said, it's best to bring your strongest physical attackers to this fight and have them attack normally while Kain uses Jump repeatedly.
Another good strategy involves using Edward equipped with speed boosting gear and using his Escape command to prevent his doom timer going down. Once everyone's been KO'ed, he will automatically return and the player can Salve Phoenix Downs to revive everyone. If his speed is high enough, it's possible for him to Escape again before his timer reaches 0. As Ahriman will not cast doom while there are still characters with timers, the other characters will be free to attack while the Ahriman casts haste on them.