(assuming this is Smash 4) I'm gonna led this off by saying I have a really weird Luigi; an optimal Luigi is aggressive and in-your-face. My Luigi is really heavy on bait-punish and footsies. I only really approach when I have to, AKA when Fox runs away and starts lasering. If you land a grab near 0 (<5%) dthrow, wait a moment for him to fastfall in hitstun, and then grounded upb for a solid 32% total. Mix up the height at which you recover (assuming you recover how most Luigi's do, sideB from edge of screen to under the stage, then double jump upB) from basically offscreen near the bottom blastzone, to just below the actual stage of battlefield. You probably already know this, but tornado had ridiculous priority. Try using it occasionally as a landing option if you are close to Fox or if he goes for a follow up. Hope this helps. If not, let me know and I'll see if I can come up with something more specific.
