Shuckle isn't in SV, so right now Shuckle doesn't exist in competitive. But it gets used in showdown formats where it's allowed. In doubles, people use Power Split to weaken opponents and Guard Split to beef up an ally pokemon's defenses.
In singles, it is almost always running stealth rock and sticky web to set up hazards (again running Sturdy for ability) and encore. Some people might use it to toxic, but it's usually running infestation so it can slowly kill pretty much anything that doesn't have the power to take it out or gets stuck in encore. A swap out to kill Shuckle activates the aforementioned hazards.
There are also meme sets around power trick to use it as a sweeper, but it requires priority and is easily shut down by priority moves because then Shuckle has paper-thin defenses and super low base HP.
