You shouldn't be aiming to break a shield. Your primary goal is to land a good hit in whatever way you can; this means either by baiting a punishable action (usually roll or sidestep but others exist too) or by doing something unexpected yet amazing (usually grab).
My thoughts exactly. The aim is not to blow up their shield with obscene amounts of tech skill, as cool as that may look. You should be looking for your opponent's responses to your shield pressure, then effectively countering them. Doing nair shine x5 is great, but if your opponent is chillin' in his shield why not just cut it short and grab him? IMO,
that's successful shield pressure. If you see them getting antsy on defense chances are you won't need to shine grab, and one of your nair/dair shines will hit for a combo opportunity. Being able to adapt to your opponents choices during shield pressure help you in all of your match-ups.
Nair shine dair shine: Mostly used when looking to interrupt an impatient opponent after the first shine. Dair can't be CCed, and yields a more consistent follow-up opportunity. If you think your opponent will try to stop your pressure after nair->shine, JC the shine into a dair and either go for a thunders, waveshine, or drill grab to keep the offense going. It's active hitbox is great for just ticking away at a shield as well, so if its blocked you still have a few good options at your disposal.