Thank you!!He has to aim for the head. I highly recommend that he uses thunder magic to bring him down. Hitting him multiple times results in mim tipping over, allowing you to hit him more until her gets back up.
Heres advice to all who are having a problem with a certain heartless:
You can always withdraw from a mission and check the enemy profiles to see what the weaknesses are. The enemy profiles are located within the files option below Roxas's Diary. You don't have to defeat a heartless to have it added to the list. After you encounter it the first time it's automatically added.