Equip Cecil with arrows in his good hand and a bow in his bad, then get in a fight. Go to items, select the arrows he has, and put them in an empty space. Without leaving the window, select the Avenger sword and move up and trade it for the bow you had equipped. Leave the item window, and his commands will disappear. Do NOT have anyone else do anything! Once Cecil has taken out all the enemies, go to the EQUIP screen and you should see the Avenger in his bad hand and a number in his good hand. Unequip the Avenger, and equip whatever weapon you want duplicated in his good hand (where the number is). De-equip, then re-equip. Voila! The weapon in the item screen will have a black box and a number beside it. 255 of them!