Can I have some decent checks/answers to Cloyster?
Cloyster is primarily used as Shell Smasher with Skill Link Rock Blast and Icicle Spear. It has massive Defense and decent Attack, and can sweep late game if not countered properly.
Smogon University lists some of its most effective counters:
Checks and Counters
Physically Bulky Water-types: Water-types that have a decently high Defense stat such as Suicune, Alomomola, and Blastoise can take a +2 Rock Blast from Cloyster and retaliate with a potential burn from Scald.
Steel-types: Steel-types can take any hit Cloyster can throw at them at +2 and retaliate, although they might not take it well after being weakened.
Fighting-type Priority Moves: Fighting-type priority moves such as Vacuum Wave and Mach Punch can easily check Cloyster, as its only form of priority is Ice Shard which won't even 2HKO users of each move.
Mixed Walls: Mixed walls such as Cresselia and Porygon2 can take any one hit from Cloyster at +2 and retaliate.
Empoleon: Empoleon resists every move Cloyster can throw at it, although the specially defensive variant has a chance to be OHKOed by a Life Orb-boosted Explosion after a Shell Smash.
to put it simply, use a special attack. at -1 after shell smash and with a BST 45 (i think) special defence, than it will be a one shot most of the time.