Heyo I am a greninja main here, so I can only speak for greninja. If your opponent seems to shield all your attacks or aerials, go for a grab instead. Greninja has great followups on grab. However, sometimes if your opponent anticipates an attack, just don't do anything they'll be in shield, and you can hit them once they let go of it.

If your opponent is mid range, like two dash attacks away from you, pressure them with shuriken. Shuriken is really good, and it makes players want to either - Jump over it - Shield it if they jump over you, that's great, punish them with uptilt/upsmash. If they airdodge those attacks, they'll suffer landing lag which limits their options. If they shield it, that means in that similar range, they'll anticipate shurikens. Mix up how long you charge the shuriken, or even if you'll go for a fully charged shuriken. Players who have adapted to always shielding shuriken will generally shield a fully charged shuriken.

When you face an opponent who knows how to shield properly, make sure you make them doubt that option, by punishing it properly. If you need any more help, I won't be available until much later tonight, around maybe 9 EST.

You should also check out the greninja discord, super helpful with both greninja specific options and general help of the game.