Just for the sake of letting people know better options...
Hold the ledge is correct, but if they are approaching from horizontal, as they are exploding, do a normal standup onto the stage. If you do it right, you will hold the ledge to cover them grabbing it, and get up in time to grab them if they go for the stage. If they are approaching with upb from the bottom, get back on top of the stage, its easier to just guard against them regaining top control. Or roll up. w/e
More options are % dependent:
Low percent, grabbing and throwing off or throwing to follow combo is best. If you are more comfortable with her off stage and keeping her from getting on and regaining stage control, throw off. If you aren't, combo her up so every time you hit her, she goes off stage. Just be aware that Marth is bad against high % opponents.
Mid percent. Grab them and throw them off or hold the ledge while they go for the stage, then jump on and dair them. It'll pop them up and you can do a nice combo across the stage or push them back off the way they came.
High percent, grab them and throw them off, or ledge wavedash and grab from the opposite direction and fthrow. Or just do a strong move. w/e
Note: A much safer option is to just stay in ledge guard range, which is just the tipper dtilt/fsmash against a normal getup from the edge. Learn this spacing and how to defend the stage and people will get ridiculously frustrated every time they are holding the side of the stage because they can't get back on.
Another note: These options change slightly if there is a platform for them to upb to. Which is most of the neutrals. But you get the picture here.
The SB from the ledge thing is not really... useful. A weak as **** SB from the ledge hitting them just puts them back in the middle of the stage. Against their shield, at best it'll reduce them to a little less than half shield and then... they go back to the middle of the stage and wait for their shield to come back to full while charging needles to throw at you as you try to get back on stage. Fail/Fail imo. Even nair from the ledge is a better option imo just because you can L-cancel it and follow. Or fair. Or dair. Or uair. I'd probably late hit uair.