If theres one thing i know about this MU, its baiting. punishing you opponent is one of the biggest and more important aspects of winning, and against mk its all the more harder, so lets get started.
retreating(grounded) b reversal works GREAT against mk(that goes for wolf as well ;p).
if mk is within a third or so of the stage of you, thats when you want to switch strategies to safe/aggressive depending on your opponent.
if you see your opponent jumping, pop up and bair can be good suprise hit, id say fair but it startsup a little slow. jab/jab cancel is a Godsend(yes, i went there). mk can upb out of the jab cancel tho, so lookout for that and if theyve done it before, be ready with your shield (R/L or downb).
if mk is gliding above/towards you, uair/nair him. remember, multihit moves > glide/glide atk(although not so much against other pits o.o). if you shield or dodge an up b and hes gliding away from you to escape, unless he's in range for a quick aerial dont try to punish with anything besides a simple arrow, maybe a jab cancel.
if you rock with the whole looping thing, make him wish he wasnt planking. if hes just kickin it on the edge you can hit his hang with an arrow. if he like to upb/neutral b.side b onto the stage, mirror that ****er to death.
generally when going under the stage to the other side, pit ***** mk in speed. if he follows you just mirror his recovery.
most mk's (aka offline competetive most brawl players) think ftilt/dtilt along with dsmash/fsmash on the ground and all aerials just **** pit. well, their right. im not 100% sure on this, but im pretty sure when landing any perfectly spaced aerial cannot be punished to a lethal extent. meaning the mk shouldnt be able to dsmash/upb before you have a chance to roll away/shield, etc. mk wants you above him, its your ground game thats going to win. so when you start to get in range with him when falling down towards the stage that although your options are severely limited, try your hardest to pick up on whatever your opponent tends to do when you land. its critical that you find a method around his douche grab and anti erotic aerials. do not air dodge/sidestep constantly, granted this is what you should do when you actually see the attack, but NOT whenever you get hit, its very predictable, very punishable, and mk of all people molests your poor angelic soul harder then most. dont throw out wild or pointless attacks, if it cant possibly hit him, dont throw it out there unless you know he cant hit you.
proper punishing: (oys-on your shield)
dash attack: if he hits with the beginning of it, he'll end up behind you, and you can pretty much reply with anything, turn around grab, jab, smash. if he hits with the end, chances are he can save himself or attack against before you can do anything. make sure your aware of this, very simple make sure your aware of mk's ending lag. generally the mk with dash attack out of dthrow, when your standing near the edge, or when landing. a quick dash attack out of dthrow can be air dodged, or you can jump. basically if you notice the mk doing dash attack out of dthrow you'll just jump, if the mk knows what he's doing he'll mix it up, so just go ahead and do the same, maybe fair or jump dair and find your way down a different way. other than that you can shield grab of course, or at higher percentages easily f or dsmash OoS for the stock.
fair: im just gonna make this one short and sweet because anymore mk's wont fair unless they know their gonna get away with it. however if they ledge hop fair nice fsmash or jab works just fine. well spaced fairs you prolly wanna just retreating arrow his ***.
dtilt: if this is one of those mk's that just spams dtilt, he's a dumb one and you can angle your shield down til he inches himself into your grab range. well spaced dtilts mixed up with ftilt, etc,---run away and b reversal arrow. his fat, mexican tilt ******ry. other then that you might be able to jab him mixups, just be sure not to get to anxious to get some dmg out of it. roll away, jump out of there, upb, whatever you got do.
ftilt: ftilt gets its own section cuz its extra homo.
upb: mentioned how to deal with this earlier, but as for the initiation of grounded upb you obviously cant outprioritze it, and i cant tell you what the mk's gonna do next, but if he goes for the glide attack, shield grab is what straight people do, so grab his gay little mask and pound some sense into him(no homo).
nado: im no frame expert on pit, but from what ive seen shielding his whole nado followed by sidestep at the end of it jab or grab work, it all depends on how the mk turns up afterwords. work with it, practice, im not gonna go TOO in depth with this one, i mean, its nado. you either get hit by it or you manage to get away scott free. if you can get a hit off, kudos.
thats all for now folks. typing sucks