I'm fairly sure that Keyblades are physical magic. Since they are connected to the heart, can spurn out of thin air, and the such.
So I'd say that the Keyblade itself is unbreakable. You'd have to do something to the Heart to actually break it (See the X-Blade shattering once Ventus vanquished Vanitas). So you'd need to be a Kingdom Hearts villain, essentially.
Sly messed Clockwerk up. He was in such a state that he alone could not reconstruct himself. Armor or not Clockwerk is defeatable, though it does seem difficult. Now killing Clockwerk is another question entirely. I think it'd be fine if Hiro does damage Clockwerk, because right now we are in a state where you like throwing villains at us who continually get away unscathed. Clockwerk will be fine and I'm sure we can see him again soon. Even if we do break him apart, nothing is stopping Specter from picking him back up once the intruders are dealt with.
I mean, it's pretty boring to fight Mr. "I can possess people with my eyes and also am INVINCIBLE!!1".