yeah, use the shl/ rshl (or even shdl/ rshdl) to make the marth come to you. they'll probably be wanting a grab, so watch out for grab setups (i.e. DD>grab, fair>grab, uair>grab,etc...). I disagree about staying off the platforms, it's important to not stay put on them for long, but you can move around on the platforms decently against marth (you might want to avoid them all together on stadium neutral though). another combo that works is the waveshine>nair: at lower %s you can nair>shine>nair>shine>uthrow>uair or something. Try dashdancing in and out of his range, bait an attack and grab (or usmash if you're close or need a finish).