Cloyster Sets and Counters
Skill Link Set
Nature and EVs: Adamant: 252 Atk / 252 Spd / 4 HP
Item: Life Orb / Focus Sash
Ability: Skill Link
Shell Break will be one of Cloyster's main assets in the 5th generation competitive scene, halving its Defense and Sp. Defense in exchange for double Attack, Sp.Attack and Speed. Cloyster already can't take special attacks well though, so a good deal of prediction is necessary with this set. With or without the boost, the moveset is pretty straightforward. Icicle Spear hits five times thanks to Skill Link and picks up a STAB boost, smashing through most enemies and ravaging substitute/subseeders like Serperior. Rock Blast doesn't pack STAB, but it gives Cloyster nice coverage and an option to use against other Water types. Razor Shell is Cloyster's other STAB attack. Life Orb sends Cloyster's power through the roof, whilst Focus Sash gives it leeway to set up Shell Break if your prediction isn't that good.
Rapid Spin Support
Nature and EVs: Adamant: 252 HP / 252 Atk / 4 Spd
Item: Focus Sash
Ability: Shell Armor / Overcoat
-Spikes / Toxic Spikes
Cloyster can make a very viable Rapid Spin user, if your team needs the support. With that obvious slot taken care of, Razor Shell stays in as your standard STAB attack. With Focus Sash at your side, you can go for a last second Ice Shard at death's door, or just use it to take out anything weak to Ice with that nice priority effect. Leftovers is just too risky on Cloyster, due to it's depressingly low HP and SpDef. Spikes and Toxic Spikes take the last slot, since this set is based on support. Shell Armor protects you from unlucky criticals, while Overcoat protects you from being killed by a Sandstorm after Focus Sash saves you.
Lucario and Toxicroak can take out Cloyster with their STAB Vacuum Wave, scoring x2 damage and hitting Cloyster's pathetic SpDef with the priority benefit. There's lots of easy ways to take out Cloyster, but this one pretty much assures success. Anything that is faster than Cloyster with Focus Blast and Aura Sphere also stands a nice chance against it. After using Shell Break, Mach Punch is a pretty safe bet to downing Cloyster. Breloom safely switches into Rock Blast and Razor Shell (Well, as close as you can get to safe), and takes it out with Mach Punch. Yanmega can use Speed Boost and Protect/Hypnosis to gain a speed advantage, then take it out with anything (It doesn't really need a type matchup, it's crazy strong and Cloyster's SpDef is usually under 100 after Shell Break).
The main thing to target is that abysmal 45 Base Sp.Def. Cloyster cannot take Sp.Attacks well, especially when it's from Grass/Electric types. Lanturn makes a nice counter in this department, since it takes everything Cloyster has except Rock Blast. Rotom W and M work well also, but W is a better option, since M is weak to Icicle Spear.
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