Shine grab. If you shine his shield, a JC grab may be better instead of starting to do pillar cross-ups or jumping away.
If the Marth just sits in shield and tries to shieldgrab you, it may not be a bad idea to just run right at him and grab him (occasionally). Yes his grab outranges yours but if he expects dash attack and waits for it, you'll get the grab. Also, if you dash attack his shield close enough and he instinctively shieldgrabs, you'll actually be behind him and can pivot grab him then (or SH aerial or shine him).
If you [accidentally?] jump and he immediately puts up his shield, you can also just land without any aerials and grab him (tomahawking can let you get grabs).
You can also go for a grab after a correctly SHFF'd laser, or really just a laser close enough to the ground - it produces enough shieldstun to give you a frame advantage so he could not just shieldgrab you. You can also do laser -> shine -> JC grab if you really want to be fancy (and more importantly, that's also very safe on shield and it means if the shine hits you can start comboing, so it really is a very strong option, not just cool-looking).
I'm sure there are a lot more suggestions to be given, but this is just a little bit from me right off the bat. Hope it helps some.