http://smashboards.com/guides/everywhere-at-once.336/
BluEG said, it's basically how to beat marth.
Other stuff:
-To counter SH aerial go WD Uptilt under them
-To counter Dtilt, go WD crouch Dsmash OR WD shield grab
-To combo, Uptilt to Upair (Trap survival DI) to Fair/Dair
-From Grab just Dthrow/Upthrow to aerial if they bad DI. Fthrow to WD Ftilt could be a thing.
-To edgeguard, Grab the ledge just befor they could reach it. Wait that all your frame of intangibility end, then react:
1)Marth is too low to get on stage = Roll
2) He's really near to you = drop Bair (do it most of time when you are still intangible)
3) between the two= (I dont have real answer for now but: ) Drop ledge, DJ Dair and regrab de ledge OR regular stand up from ledge to Ftilit/Crouch Ftilt/WD back to the ledge.