Donald and Goofy can't kill it, it doesn't take damage. It appears, and then disappears immediately until you do a specific thing
In the Bazaar in Agrabah, three will appear, and then disappear and hide in one of the stands, jsut use Fire to plow through them all and they'll appear.
In Christmas Town in Halloween Town, it'll appear and disappear, and then it will pop back out as soon as you go up to the Carossal an use the reastion command to make it spin
In Beast's Castle, in the West Wing/Corrider, it will appear and then hide itself in a Suit of Armor. Use Magic to destroy all the suits of Armor and it will apear
In the Underworld, in the place you first saw Demyx (Circular Room with a circle of those lamps in the middle), it will appear, and then hide itself in one of the lamps. Destroy the lamps to have it come out
In the Land of Dragons, at the place where the Army Camp, Emperial City, and Mountain trail meet at that river place is where the last one appears, you have to destroy one of the Missile carts for it to come out
After it appears, you can't damage it since it's immune to everything, but it's health will go down naturally. The less health it has, the better the prize you get when you do the Reaction Command, so remember to wait until it has 1 bar of health left, or less than 1 bar of health
