@ King's Rock
Trait: Skill Link
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature (+Atk, -SpA)
- Shell Smash
- Icicle Spear
- Rock Blast
- Ice Shard
Posting this before Lavos. Although White Herb is usually the standard item on cloyster, I rarely find it useful. Even at -1 / -1, basically no non-SE physical moves or OHKOing (and you shouldnt be setting up on terrakion, for example. Meanwhile, most decently powerful special moves are OHKOing even without the drop. Additionally, cloyster is fairly slow before a SS, so most of the time you'll take a hit
before setting up, meaning priority is really the only draw of white herb, but this is fairly rare and cloyster can take a hit at -1 from basically every priority attack bar techniloom MP, which already KOs after rocks even with WH (252+ Atk Life Orb Technician Breloom Mach Punch vs. 4 HP / 0 Def Cloyster: 172-203 (71.07 - 83.88%) -- 62.5% chance to OHKO after Stealth Rock). So the better choice is to choose an item that helps cloyster to sweep. Cloyster's biggest problem is that many buliky mons (bulky waters especially) are only 2HKOed even at +2. LO doesn't help much, as it's not quite enough of a boost. The solution is king's rock. With skill link, cloyster has two 5-hit moves, so the chance of getting a flinch each turn becomes more than 40%. Given that +2 cloyster 2HKOs but doesn't OHKO the majority of the meta, this is a huge bonus, and could easily turn the game around. Meanwhile, Ice Shard allows cloyster to beat stuff like scarf latios, which would otherwise outspeed and KO it, allowing it to run adamant for more power.
Finally, if you're not convinced yet, just check out
this WS to witness KR cloyster's awesome power.