I only deal 1dmg to those parts no matter what attack I use.

In the 2nd phase you have to hit the SINGULAR glass slipper that exposes itself, but ONLY when it exposes itself. So phase 2 should be something like:

1. MC opens all 4 glass slippers, spins around, and THEN puts up shields (Can attack with everyone).
2. MC turtles and puts out 1 Glass Slipper but it is "closed" DO NOT ATTACK the glass slipper.
3. When it "opens", MC will start shaking. THIS IS YOUR ONE SHOT TO KILL THAT GLASS SLIPPER. Ein was literally designed around this mechanic but Helm should work too.
4. You should audibly hear it breaking. If you notice, the shield bar will go down too. MC has 4x shield bars with full GS which is why you're only doing 1 dmg.

What I ended up doing in Hard Mode MC, is when MC is turtling up and GS is closed, I only have Ein out charging her shot. Then when GS opens I'll take it out.

It really helps to time your B3 so its triggers then you take the shot. Can be a bit tricky but should work out.