Learn how to lightshield edgehog which makes marth edgeguarding 5x easier than it already is. All you gotta do is roll to the edge, lightshield and hold AWAY from the middle of the stage. Don't hold down or you will not grab the edge and be sent to ur death
other ways to edgeguard:
if they are coming from below, you can just jumpoff fastfall b-air, if you do it correctly at worst you will trade with up-b
if you lightshield and they are sort of high or next to you theres a 90% chance that they will airdodge, keep that in mind and jump out of shield with a b-air or something.
Marth's might up-air early (and still grab edge if not hit) if they see you lightshielding, just **** them with an aerial out of shield
Marth isn't difficult to edgeguard and once u can edgeguard 99% of the time matchup is way easier