if they are scared and shielding you can have some fun mixups
grab is one of those. another is sh kick
another is run into the middle of their shield then upsmash LOL
personally i try to get inside their head as much as possible, and when that happens, thats when the grabs come out
but if they aren't shielding/scared, chances are the grab wont work too well... its like the samus grab, you gotta get them afraid first.
for approaching i use dash attack, a lot of dash dance/long wavedash stuff to feel them out, grab if i think they are going to shield, and retreating/approaching fairs and bairs.
edit: take notice of what they do — if you find your dash attack getting shieldgrabbed, perhaps that was a good time to try something else like a grab or a spaced fsmash.
sometimes i throw out Fsmash incase they run into it (it can happen)