Depends. If they're just perfect-shielding everything ever and dodging everything that can't be shielded, there is absolutely nothing you can do about that; a perfect shield is absolutely 100% safe from everything that's not a grab or Little Mac's Guard Breaker custom move.
If they're just sitting in shield, feel free to hit it hard with a properly-spaced Smash Attack, or simply dash-grab, roll-cancel grab, or dash attack-cancel grab (whichever gets you to the target and grabbing them fastest). For example, Mii Swordfighter's FSmash is pretty much safe on shield if spaced properly, because they'll be pushed so far back that they won't be able to do anything before you can act out of it. If they dodge or perfect-shield, though, they get a free punish. So feel free to hit shields aggressively in the hopes of getting them to breaking point, just as long as you do it from a safe distance.
Edit: Other moves that smite shields effectively: Bowser Bomb is an instant break, supposedly DK has moves that can instantly crush a shield, fully-charged or near-fully-charged Shield Breaker from Marth or Lucina is an instant break, Warlock Punch from Ganondorf is almost certainly an instant break (and it's super-armoured too), and Ganondorf's DAir hits shields stupid-hard as well (though it's not safe by any means unless you actually get a shield-break with it).