I have a few strategies that I've developed to give marth one hell of a fight. I usually beat marth's with my game and watch.
to perform the following strategies, you must be able to execute all of the advanced techniques. i.e. triangle jumping, wavedashing, l-canceling, wavelanding.
strategy #1 :
What G&W does: Fake an approach with a short hop, at your peak DI backwords.
What Marth does: hopefully he'll try a fair, or better yet, an fsmash.
if this happens, keep DIing backwards so he misses. execute a wavelandfsmash, or dsmash.
or just land, and to a quick fair to catch him off gaurd.
Strategy #2:
What G&W does: after marth lands on the ground GW comes flying in with a bair, this move has the most range and can hit marth before he can smash or tilt if you approached quick enough. the move has little knockback, you should land near him.
Little knock back is what you want, l-cancel and throw a c-sticked smash attack at him. This all depends on where you land in context to marth. if you land on him, do an usmash fast
land close to him do a fsmash or dsmash. I like dsmash if I'm close, because if I don't sweet spot the hammer, then he goes almost no where, so then I do a fsmash, fair, or dtilt. (or judgement if you are feeling lucky)
you want to get marth in the air, this is important. G&W has great uair priority on marth
jump uair, jump nair.
strategy #3
what G&W does: toss 1 bacon,
what marth does: he either shields, wavedashes backwords, or tries to hurdle it. if he swats it then plan fails.
what G&W does: if he shields, rush in with a dtilt, or a flying key attack, both are great for penetrating the shield. always l-cancel aerials.
if he wavedashes backwords do strategy #1
if he tries to hurdle or jump the bacon, you smack him with a fair. hit him inbetween his fairs, or beat him to it.
after this, throw a nair to get him off the level
ledge gaurding:
fsmash works well if you crouch the upb
time a shffldair on the ledge, the hit box will get marth and it will either hit him up and out, or it will be a meteor smash.
if he's trying to come down from a platform, he's vulnerable to utilts, and wavesmashes.
always be shorthopping and wavelanding to get marth to make his move first.
good luck fellow game and watchers, may the bucket be with you.