April Fools. It is very difficult to consistently beat Luigi's misfire with the strong hitbox of Marth Up B here. My actual recommendation is to Marthkiller > drop shield > tipper Dsmash. You can react to no misfire > short hop out of shield > grab ledge from this same position.
Marth can shorthop > react to audio cue > double jump reverse Up B to cover Luigi misfire. This presents an alternative to timing Fsmash early, which can whiff if no misfire. The Up B puts Marth on ledge afterwards and ready to continue the edgeguard if Luigi does not misfire.