GIF needed
By holding a light shield at the edge angled off-stage, any attack will trigger shield ASDI, sending you to the ledge. Being a fastfaller, Fox will always grab the ledge faster than the assailant.
This option is comically oppressive against Marth's recovery. While he can hit it with an aerial, recover high, or double-jump to ledge, the mere threat of being Marth killered forces him into a mixup game that is rigged in your favour.