You can't fight MK if you don't know how to FFAD (fastfall airdodge). For MK, you space with SH dAir FFADs. Polmex and Boss do it habitually, but I was surprised to find nobody mentioned it yet in this thread.
MK loses a lot of momentum when he shorthops in the air, so if you space, the only attacks that should reach you are the ones he does just as he runs up to you for a grounded or SH attack. You keep him and those attacks at bay with spaced SH dAir FFADs, just like Polmex and Boss have shown in their videos. You don't approach with it, you just space (except when you want to mix it up a bit)... MK's do all the work for you and just run INTO the dAirs like moths to a flame. If you do approach with it, you might as well either FF nAir instead of FFAD, or follow the FFAD with a buffered Jab (MK doesn't give Luigi any large time windows to attack).
If their approaches aren't working as well and begin to get too defensive, SH dAir to Grounded Cyclone into/past MK. Like with Falco/Fox, the SH Aerial to Cyclone is actually pretty effective against MK even though he has disjointed hitboxes.
MK's attacks deal relatively more knockback than damage anyway. Luigi will get KO'd more by getting gimped while recovering than from getting uncontrollably launched offscreen. You won't die from percentage. So those little attacks and Mach Tornadoes are just dust on your shoulder. It's his Shuttle Loops, dSmashes, etc. that are dangerous... because he chases you off stage.
So, inevitably, every Luigi here that wants to improve his game against MK needs to recover better. On the other side, MK does die from percentage. It's being at 60% and seeing you wildly throw fSmashes around like grenades that scares him.
Man, I didn't plan on writing this much. I need a summary of what I said now: Space with SH dAir FFADs. SH Aerial to Cyclones and FFAD Buffers work well if you wanna be aggresive. Don't sweat his small attacks... just dust your shoulders off and continue spacing. Improve your recoveries. What else? Oh yeah! fSmash if he's too agresive. Like a madman. Good times.
. this is one of the match ups i really need to become great with before whobo
