GIF needed
It is possible to ledgehog and threaten Marth killer. If you have taken ledge to counter Marth's double jump or after being hit out of Marth killer, you can neutral get-up, buffer light shield, then shield SDI his up-B.
One shield SDI input is enough to put you on the ledge; the window is five frames long. Possibly the best part of this technique is its low risk and high reward; success means taking a stock whereas failure just gives Marth the ledge while you have the stage.