Also, remember a few things NOT to do.
*Don't spam n/b-air approaches...Marth's sword is too long, and you'll just get f-smashed. Instead use pills to make openings, or trick him into f-smashing, while using your other jump to get over/behind him and punish the missed f-smash.
*Try to roll towards him when thrown: Usually they can hit you with the f-smash's tip is you roll away. Rolling away seems to be a natural reaction, but quelling it is useful, as you then get hit by the base of his sword, which isn't as powerful, or ger behind if chases your tech the wrong directions.
*Don't be predictable (semi related to previous note)
* if you hit him, make sure you then proceed to follow up with SHFFling aerials and racking up damage or smashing him. Don't connect and then run away/give him enough time to recover.
*if the marth spams SHed n/f-air approaches, don't just try to shield grab them...good marths can space themselves and then punish an atempt to grab. As both aerials leave marth's lower back vunerable, try dashing under/WDing under, and then attacking their back.
*when edgeguarding:
-- jump out at them before they have time to drop below the course and sweetspot the edge.
--don't spam pills, they usually won't reach, and will just aid the recovery by popping them up some and letting them up-B again. Instead, b-air works wonders.
--useful trick: turn away from the edge and lightshield. when Marth up+B's, it should push you off the edge onto the ledge and autoledge guard him. <--not sure if this is accurate, but something close to it.