Shuckle has multiple hard counters, but most of them tend to be specific moves rather than Pokemon. Taunt or Trick renders Shuckle useless, and Substitute prevents it from doing anything. Any status absorber, especially RestTalk users, will render Shuckle relatively useless. Stealth Rock is another good check to Shuckle, who hates losing 25% of its health every time it switches in. Setup sweepers are also good checks, as a single misprediction can cost the opposition the game. Setup sweepers with Substitute are the best at setting up against Shuckle as they don't fear its Toxic. Also, recovery moves and careful switching prevents the Toxic counter from stacking too high. Finally, Steel- and Poison-types such as Probopass and Muk can switch in easily, as they are immune to Toxic, and proceed to do whatever they want.