The Undying has no life-bar, so the player may not know how much damage he has taken. He has Bahamut's signature move, Mega-Flare, which deals above 3000 damage to the entire party. Most of the time, he will cast piercing magicks, as well as dispelga, making most buffs useless. After some damage, he will cast a magick barrier, negating all magick damage. After more damage, he will cast perfect defense, negating everything thrown at him, then buff himself and use Giga-Flare. After his protection ends, he will start healing himself, then use Tera-Flare, his strongest attack.
Vaan's party defeats The Undying, who explodes in a burst of mist that covers the entire Sky-Fortress, which falls apart and crashes in the desert outside of Rabanastre.