Versus Fox is tricky since he can shine kill you once. My advice is jab. Jab dominates, and if timed correctly can go through his NAir approach. You could always just roll from the ledge, it works if you've been wavelanding from the ledge every time and they know it's coming. Also ledge hop ~> jab/grab is a good approach as well if they attack the ledge expecting you to come up and you time it right.
Nair has its uses but it is limited. It's good if you get a grab on someone like Jiggs/Samus and follow up dthrow at like 0-percent because both kicks land(depending on DI). I find it useful when the opponent is at low percent, because depending on DI it can set up for a double jump aerial. Since it's unexpected, it also can steal their double jump which in turn leads to a free follow up hit. Kage may use it a little differently, I'm not sure.
