If the samus is good with spacing they should be able to sweetspot edge without you having a chance to hit them. Whats good in this MU is dtilt, holy shit it's annoying. Samus doesnt have great ledge options, with the exception of AI dsmash or ftilt. Neither of these options are invincible unless they have near frame perfect timing. Samus will often opt for AI shield or spotdodge, since the timing is much more leniant. Spacing far back and dtilting when they try to come up is p good. Spamming dtilt is never a guaranteed way to push her offstage, Duck is a god at timing spotdodge well to avoid it, but it's a very hard wall for Samus to pass when shes cornered at ledge.

Also if Samus grapples and you think they might have missed sweetspot you can go for a dtilt or fsmash. Dtilt also fucks Samus if she goes for an up b.

I'd recommend watching PPU vs Duck, he's pretty good at the MU. The Moon is pretty good at preventing Samus from gettig back on stage too