Technically, Bullet Punch cannot knock out Shell Smash Cloyster in one hit calculated by the Pokemon Damage Calculator
252+ Atk Choice Band Technician Scizor Bullet Punch vs. -1 0 HP / 252 Def Cloyster: 126-148 (52.2 - 61.4%) -- guaranteed 2HKO
But if you give Superpower to Scizor, it can one hit knock out Cloyster
252+ Atk Choice Band Scizor Superpower vs. -1 0 HP / 252 Def Cloyster: 332-392 (137.7 - 162.6%) -- guaranteed OHKO
Aegislash
Another strategy is to get lucky with Aegislash, if Cloyster decides to go for Liquidation twice, then both King's Shield managed to not fail, this should be possible
0+ Atk Cloyster Liquidation vs. 0 HP / 252 Def Aegislash-Blade: 114-135 (43.6 - 51.7%) -- guaranteed 3HKO after Leftovers recovery
While we do
252+ Atk Aegislash-Blade Sacred Sword vs. 0 HP / 252 Def Cloyster: 118-140 (48.9 - 58%) -- guaranteed 2HKO
Here is the set:
Aegislash
Ability: Stance Change
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- King's Shield
- Sacred Sword
- Psycho Cut
- Shadow Claw
Arceus-Rock
This would be too easy to do
252 SpA Stone Plate Arceus-Rock Judgment vs. -1 0 HP / 4 SpD Cloyster: 1038-1224 (430.7 - 507.8%) -- guaranteed OHKO
But Cloyster would move first
+2 0+ Atk Cloyster Liquidation vs. 4 HP / 252 Def Arceus-Rock: 270-318 (70.6 - 83.2%) -- guaranteed 2HKO
Here is the set:
Arceus-Rock @ Stone Plate
Ability: Multitype
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk
- Judgment
- Draco Meteor
- Fire Blast
- Recover
A Simple Answer
Mostly Pokemons with high defense, super effective against Cloyster or Pokemons with high special Attack
I hoped I've helped!