wel, you just make sora die, that's all (by being open to attack).
i haven't tried it in any other battle, except the demyx battle in which mickey DOESN'T come, but this is what you can do as mickey:
O--attack (powerful swinging attacks)
X--jump
SQUARE--shoot a ball of magic, the same one minnie does (you don't command minnie to do it, she just does when heartless are nearby) which depletes MP.
i forgot which button this is, i think triangle, but if you KEEP ON PRESSING IT rapidly, the drive meter increases. once the drive meter reaches MAX, you must find sora in the area and press triangle when near him. this revives him, and you must use sora to finish off the boss. you cannot finish off bosses with mickey, but you CAN deplete the enemy's HP to 1.