Finally, a non-unversed boss. This fight is actually pretty cool IMO. It's easy, but cool.
Most of this bosses moves are easily dodged. It tries to throw you off with making duplicates of itself, but it's easy to figure out what to do. The boss does the following attacks:
1. A row of faces appears. They shoot red projectiles. Just run/slide down the path to avoid, it'll eventually stop. If you happen to see one smiling, he's the real Magic Mirror and can be attacked to stop.
2. A circle of faces appears around you. They spin and stop to shoot red projectiles. Slide to avoid. The real magic mirror can be targeted and attacked shortly after the first round of projectiles. Attack him to stop it.
3. He'll move back and forth approaching you slowly for major damage if it hits you. Guard this to stop him. Be very careful not to stop Guarding too soon or you could still get hit.
4. He'll fly around and shoot a projectile every so often. He'll probably do this 4 times. Just Slide around to avoid.
5. He'll sit back in the distance and then come charging at you full speed. Guard to stop him. Be careful of this one.
After each of these moves, he should be wide open. Any hits from an attack stun him so you can fully combo him safely. If you hit him hard enough, he collapses for another free combo. By this point in the game you've probably started Melding some nicer stuff, so don't be afraid to use it.
The boss is weak to Stun, Poison, and Sleep. I find it hard to hit him with Sleep, I wouldn't bother. Poison is probably pretty good, though. Just be careful of your aerial combos not finishing fully on him. My combos had a issue with fully connecting due to him moving around during getting stunned.
Q: What's CM?
A: Critical Mode, it's the hardest difficulty. You deal less damage, take more damage, get less HP, but start off with more Command Slots.
Q: Hey, at the end of some fights you don't have full HP! Doesn't that mean you got hit at some point?
A: Some boss fights give HP Bonus as a reward. This increases your HP instantly, and your HP does not fill up. It's easy to notice this just by simply re-watching the end of the fight.
Q: Why not LV1?
A: Don't feel like it. The LV1 fights in this game take much longer than in KH2FM, and most people probably don't want to watch fights that take 10+ minutes. Also, it's not as hard as in KH2FM because you can earn the Second Chance/Once More abilities at LV1 as well as all abilities, so it's almost identical to a low level CM run (what I'm doing), only that you deal much less damage at LV1 unlike in KH2FM.
Q: How come you sometimes don't get hit during moves.
A: Certain moves give you invincibility frames until they finish, such as Finish Commands and Shotlocks. However, you're wide open afterwards.