Guildenstern is the final boss of Vagrant Story. This is his second form, created after his human form is defeated. In classic Final Fantasy fashion, this form has the appearance of an angelic monster. Guildenstern's HP and MP stats are 666, in reference to the Christian number of the devil.
Guildenstern is incredibly difficult to defeat. The battle will begin on a magic platform on top of the ruined Cathedral. Guildenstern floats around this platform at a very fast speed, casting both Light and Dark affinity spells at Ashley.
The only way to attack him is to wait at the edge of this platform until he casts a status-changing spell on you. Before he casts it, you will have a chance to attack him (you will most likely be doing 0 or 1 damage). Use a Heavy Shot and Crimson Pain chain combo to deal the most damage, and use a Vera and Cure potion to heal after this. After some damage is dealt, he will fly up and you will have a chance to attack him as he flies over your head. If you fail, he will execute his ultimate attack: Bloody Sin. The only way to defend against this attack is to use Demonscale as a Defense Ability, though timing it can be rather difficult.