hi, long time lurker first time poster. i feel like i have to post about this matchup, because it gave me such stress when i was first learning it. my friend mains meta and has won some tourneys in our area, i main g&w (though i'm learning lucario, lucario is so overlooked). anyway, we play each other a LOT... like tens of thousands of games o_o at first i couldn't touch his meta, but lurking smashboards+watching youtubes of some top g&ws really helped.
imo this matchup is all about experience--you have the tools to counter every single one of mk's moves, but it's not easy, as his moves are faster, less punishable and have comparable priority/range. on the other hand he can punish you easily if you whiff, at the peak of your up b, or if you get too predictable... for instance if you spam SH bair as an approach, an experienced meta will just tornado or shuttle loop through it.
some basic stuff is, spot dodge immediately whenever you get launched into the air, spot dodge whenever you are directly above meta in range for a shuttle loop or rising uair/nair, spot dodge whenever meta lands behind you in range of his dsmash, never approach a gliding meta head on except with a bair. this will negate every single one of his kill moves (except downb, lol) and make him work hard to finish you, whereas your dthrow chains, fair out of shield and even stuff like dair and upb off stage have tremendous kill potential and are easy to pull off.
anywho, here are the approaches good metas will use:
walk forward to ftilt - this outranges every move you have... not even bair will go through this. if you know meta is going to do this, you can wait, shield it, then fair out of shield after the third hit, so that you hit him during his animation lag. however, meta can cancel or slow the second and third hit, so be on guard for that, and vary your timing... if he doesn't do the third hit you can calmly walk up and grab or chair him in the face. be wary for this at all times, since it's meta's safest and easiest move for punishing or approaching you on the ground.
dash attack - this move is deadly because it is fast, hard to punish and leads into strings of attacks. be sure to airdodge right away if you get hit, to avoid an uptilt, nair or upb followup. even if you block, meta will end up behind you, and if he's still in range, he'll probably follow up with a dsmash. a good thing to do if you block/dodge this is to jump forward somewhat, then drift backwards with a bair. never try to retaliate on the ground because metaknights always follow this move up with something, and his speed plus proximity will own you.
dash grab, or walking grab - this comes out fast and has a lot more range than your grabs, especially the dash grab. good metas will try to grab you right after you land from an aerial (except nair), during the split second of landing lag... or during animation lag on your dtilt/fsmash... or if you shield too much. jump more, or use upb right before or after you land, to avoid getting punished by this.
mach tornado - your fsmash, dsmash, dair and dash attack will go through this, sometimes even a bair if you hit near the top of the tornado... but in each case the spacing has to be very precise. unfortunately a good meta will only use this to catch you off guard, when countering would be difficult. if i don't think i can land an fsmash, what i try to do is play keepaway, staying just outside the range of tornado, then punish with an aerial or smash just as meta comes out of it. you can also try upb to blow mk upwards and into special fall, though i've never had luck doing this.
shuttle loop - this move is complex and powerful, but with proper spacing and presence of mind you can break it pretty easily. if you're close to meta when he starts it up, like if you shielded the opening frames, you can interrupt it with a quick nair or uair. you can also dair the loop from above, IF you time it right. what you want to do is thrust your key into meta's belly or legs as he loops. a split second too soon, and he'll loop into your side as you fall, causing you to be hit outwards. as for the glide part of the loop...
glide attack - there are two safe ways to counter this. if you're a good deal below him, you can use the uair breeze effect to screw up meta's flight path. you might even cause him to go into special fall, but if he's smart he'll usually end up gliding past you, at which point you can go for a bair or fair. if you're in front of meta, you can drift a bair into his glide, and it'll eat right through... make sure to aim your turtle so it hits meta, though, or you'll just get slashed.
aerial spam - this is where mk gets all up in your grill with a constant stream of fair, dair and nair. trying to fight this head on sucks, as meta's moves will hit before yours can come out, and they just keep coming. upb won't always save you, as if you miss you'll probably be eating a shuttle loop, nor will nair outprioritize meta's fair... use your head, take advantage of spot dodge, and get back out to middle range, where your bair dominates. don't succumb to the pressure.
spotdodge into dsmash, upb, grab, or whatever - this is really a part of mindgames, but i feel like mentioning it because it's really one of meta's more effective KO techniques, due to the speed of his dsmash and upb. he'll either walk or run up right into dsmash range, spotdodge in place, THEN dsmash (or upb, or grab, etc) watch out for this, especially when you're at high percents. your fsmash's extended hitframes will own the spotdodge if you can anticipate it. or you can charge a dsmash... just be aware of this sneaky tactic and you can usually counter it.
that's basically it, i think. learn to counter each of these approaches and you'll have neutralized the bulk of mk's offense.
as for your own moves, so long as you keep meta out of your face your aerial strings will own him, as will your upb. try to predict when he'll airdodge and attack before or after. bair will chew through everything meta has except tornado and ftilt. nair is terrific for attacking from below and at close range, uair refreshes your decayed moves and is good for mindgames, fair has insane killing potential, especially off stage. once you've put the fear of aerials into meta, he'll shield more, opening up your sick, sick grab game. and once you have him offstage, you really do have the advantage in edgeguarding, thanks to your greater range and fireman's insane priority. abuse this whenever possible.
as for the ground, your dtilt will stop most of meta's ground approaches cold. anything out of shield will stop his ftilt approach. the super armor on your fsmash and upsmash make them perfect counters for meta's drill and tornado, and ther timing makes them great for punishing spotdodgers. still, on the ground i'd have to give meta the advantage. he moves FAST on the ground, has great grab range, ftilt range and a blazing fast dsmash, plus an uptilt that will break your dair... just keep him in the air whenever possible.
really, on paper it looks like g&w has the advantage, but in reality vs a metaknight who's constantly dashing back and forth, pressuring with ftilt, aerials, grabs, dash combos, dash to dsmash, spotdodge dsmash, using b moves to stay unpredictable... you'll need mad reflexes and a knack for reading your opponent to keep up. 50/50 imo.