>>17126614People use Cloyster wrong. White Herb is shit. Focus Sash is shit. Hydro Pump/Razor Shell is shit. King's Rock is shit.
Always run Ice Shard and Life Orb. You'd be surprised at how many easy OHKO's it can get.
Back in BW it would destroy sun and sand teams on the high ladder by itself.
I haven't used it much in XY but looking at some calcs it has potential:
+2 252+ Atk Life Orb Cloyster Icicle Spear (5 hits) vs. 252 HP / 0 Def Aegislash-Shield: 160-195 (49.3 - 60.1%) -- approx. 90.2% chance to 2HKO after Leftovers recovery
252+ Atk Aegislash-Blade Shadow Sneak vs. -1 4 HP / 0 Def Cloyster: 72-85 (29.7 - 35.1%) -- 16.6% chance to 3HKO
+2 252+ Atk Life Orb Cloyster Icicle Spear (5 hits) vs. 252 HP / 0 Def Thick Fat Mega Venusaur: 405-480 (111.2 - 131.8%) -- guaranteed OHKO
+2 252+ Atk Life Orb Cloyster Rock Blast (5 hits) vs. 252 HP / 0 Def Rotom-W: 300-355 (98.6 - 116.7%) -- approx. 93.8% chance to OHKO
White Herb is useful only against Smogonbird since it lets Cloyster avoid the OHKO from CB Brave Bird.
Otherwise LO is the way to go.