I don't know if this has been talked about yet... but my friend and I have been playing the Luigi vs. Metaknight match up and Luigi's Usmash stuffs MK's tornado pretty often.
Luigi has very limited options in this match-up because his strength is aerials, and MK has even better aerials than Luigi. You have to play really patient and look for clear openings; if you try to go toe-to-toe with him then you'll just get beaten out by his superior speed and range. Luigi's tornado definitely catches people off guard, you can spam it, and it can lead into combos/setups if you execute correctly. Also use fireballs when at a distance, because even if they don't do a lot of damage, you're controlling space and you can bait him to come in close. In the air, Luigi's only move that competes with MK is the nair, and if he's good then you probably won't even be able to land that very often so don't try to rely on it.
Metaknight is best when his sword reaches just outside your hitbox range, because he can just keep dsmashing/dtilting/sh.fairing to keep you out. So generally speaking, you should try to be either far away from MK where you can fireball/tornado, or up close where you can land combos (Up B's and fsmashes will be really important for killing).