Instructions
Begin the battle near the Windmill Shed. Move your Knight and your Thief to Wiegraf's position. Have your Knight attack him with Weapon Break. If that doesn't work, have your Thief attack him with Steal Weapon. Stealing Wiegraf's Holy Sword will make the battle much easier, since it is his most powerful means of attack.
Attack Boco with conventional attacks until you've defeated it. Boco is the Chocobo in Wiegraf's party, and it is the only one with the ability to heal his party. Boco doesn't have many hit points and will go down after you attack it a few times.
Cast the Bolt 2 spell on Wiegraf. He is vulnerable to electric attacks, and any character who has this spell should cast it on Wiegraf as much as possible. Attack Wiegraf physically with every other character until you defeat him. Wiegraf will retreat when his health is at roughly 35 to 40 hit points.