All Pokemon have their own downfalls. Shuckle's problem is that its HP stat is rather pathetic, so its overall bulk isn't as high as it seems. Assuming that Shuckle does not have Sturdy, a strong super effective hit can knock it out. Here are a few examples:
252+ Atk Mold Breaker Rampardos Head Smash vs. 252 HP / 252+ Def Shuckle: 248-294 (101.6 - 120.4%) -- guaranteed OHKO
252+ Atk Choice Band Tyrantrum Head Smash vs. 252 HP / 252+ Def Shuckle: 296-350 (121.3 - 143.4%) -- guaranteed OHKO
252+ SpA Choice Specs Keldeo Hydro Pump vs. 252 HP / 252 SpD Shuckle: 252-296 (103.2 - 121.3%) -- guaranteed OHKO
Those are just a few unboosted Pokemon that can knock it out. If a Pokemon acquires the correct boosts, they can easily knock Shuckle out due its inability to do anything except passive damage, unless it boosts its defenses by Contrary Shell Smash.