Shuckle

Type Tier
Bug / Rock UU
Statistics
Stat
HP
20
243
Atk
10
118
Def
230
558
SpA
10
118
SpD
230
558
Spe
5
108
Name Item

Wrap

Leftovers
Moveset
~ Toxic
~ Wrap
~ Rest
~ Encore / Protect

Looking at Shuckle, you see massive defenses... and nothing else. Its HP is in fact the lowest of all fully evolved Pokémon and tied for second lowest in the game (it's higher than Diglett's). Basically, Wrap something non-Steel / Poison (obviously non-Ghost too) and then Toxic them, praying they don't have Rest (but it's GSC, so they probably will). Use your high defenses to tank hits in the meantime, then Rest when you need to. Encore is pretty cool, as even with Shuckle's low Speed you can Encore stat ups and switch to some setup Pokémon or phazer. Or just Encore an Earthquake and switch to a Flying-type, just don't expect it to be as useful as, say, Jumpluff. You can use Protect for even more PP wasting and Toxic stalling. Still, fairly useless.

Other Options

Sweet Kiss works for confusion, possibly over Encore in the first set to be more of a general annoyance. Attract could also be used in the capacity that confusion would be, just to be more annoying and maybe stop things from hitting you / setting up on you. Safeguard let's Shuckle be more of a help to the team, and Sleep Talk because Shuckle has the defenses to easily Sleep Talk, but seriously, what can you do except stall?

Opinion

Best defenses in the game, that's a fact. Fairly good typing. So why the hate? Well, terrible HP and the inability to effectively attack at all really hurt Shuckle's usefulness, not to mention the fact that Shuckle's most effective method of damage is easily countered by Steel and Poison Pokémon, who are very common, and the amount of Heal Belling and Rest + Sleep Talk users in GSC isn't helping either. Things can easily set up on Shuckle, especially something like a Curse / Return / Rest / Sleep Talk Kangaskhan that can set up after being hit with Toxic and just rest it off, unless Encore lasts an incredibly long time. Best used on stall or TSS (Toxic / Spikes / Sandstorm) based teams, because it won't do much besides stalling.

Counters

Any Steel or Poison Pokémon, especially Gengar and Steelix, will be immune to anything Shuckle can pull off. Most Pokémon that setup can use Shuckle as a free opportunity, but they have to beware of Encore. Water-types with high Special Attack can hit Shuckle on one of its main weaknesses. Rest and Heal Bell in general will keep Shuckle from racking up Toxic damage. Anything with Seismic Toss or Night Shade bypass those high defenses and go straight for the garbage HP.