In BW, Cloyster received a gift it could only have dreamed of in previous generations: Shell Smash. With one of the best boosting moves around, Cloyster now finds itself being able to perform an entirely different role as a quite threatening sweeper around. Taking Cloyster's Skill Link ability into account, it essentially has a 125 Base Power Icicle Spear and Rock Blast, which at +2 Attack will destroy anything that lacks a resistance to it or great bulk. Its Water-type move of choice also makes sure that Steel-types aren't surefire counters, as a +2 STAB Hydro Pump or Razor Shell will inflict huge damage to most of them.
However, Cloyster isn't without its flaws. Its Ice typing provides it with a crippling weakness to Stealth Rock, as well as Mach Punch and Bullet Punch. With Breloom and Scizor being so prevalent, a weakness to priority is a major issue. Cloyster also has trouble getting past bulky Water-types, such as Jellicent and Vaporeon, that can burn or phaze it and thus prevent a sweep, as well as Keldeo, one of the most threatening Pokemon in the OU tier. All in all, Cloyster takes some skill to use effectively, but when set up, it is extremely dangerous.
Cloyster's main set is one that can sweep most teams with ease; in fact, it's a liability to have a bulky Ground-type when your opponent has a Cloyster. This set is best used on an offensive team, where other sweepers can open up holes in defensive cores for Cloyster to take advantage of. Shell Smash doubles Cloyster's Attack, Special Attack, and Speed to threatening levels. Icicle Spear is the perfect move for Cloyster; because of Skill Link, it will always hit five times, effectively making it a 125 Base Power STAB move. Rock Blast offers great coverage with Icicle Spear, hitting Fire-, Water-, and Ice-types for at least neutral damage.
In the last slot, Ice Shard is preferred to prevent Choice Scarf Latios and Breloom from revenge killing Cloyster, as well as providing a useful priority check against Landorus-T, Tornadus, Thundurus-T, and Dragon-types. However, you may also choose a Water-type STAB move to hit Steel-types, among others, hard. After a Shell Smash, Razor Shell OHKOes Tyranitar and Heatran without rain and Scizor with rain after Stealth Rock damage. Razor Shell also inflicts the most damage to specially defensive Jirachi, and it can wear down physically bulky opponents with its 50% chance to lower their Defense by one stage. Hydro Pump is the preferred option for getting past Skarmory, Forretress, and non-specially defensive Scizor since it hits their weaker Special Defense; however, it loses out on taking out specially defensive Tyranitar and Heatran with Stealth Rock support.Team Options & Additional Comments >>>
Explosion could see use on one of Cloyster's sets, but with the mechanics change in BW, it is much less effective and thus better off not being used. Cloyster can potentially use a defensive utility set with Rapid Spin and Spikes support, but Cloyster's terrible Ice typing undermines this role. Also, a Choice Scarf set can come in handy for checking faster threats such as Dragonite, Virizion, Tornadus, Latios, and Infernape, among others. However, it is generally not recommended, as Cloyster's lack of power is usually its downfall without Shell Smash to boost its stats.
Checks and Counters
Being part Ice-type is a double-edged sword for Cloyster. On one hand, it gains an awesome STAB move, but on the other, it takes 25% damage from Stealth Rock whenever it switches in, and is liable to being hit by both Spikes and Toxic Spikes. For this reason, having Stealth Rock on the field is a good first answer to Cloyster. A Bullet Punch from Choice Band Scizor will 2HKO Cloyster after it has taken Stealth Rock damage, while the only thing Cloyster can do back is Hydro Pump, which fails to OHKO Scizor even at +2, leaving aside the fact that it has shaky accuracy; if rain is up, however, Scizor will be easily OHKOed. Mach Punch users are also Cloyster's worst enemies; Life Orb Adamant Breloom has a fairly high chance to OHKO without Shell Smash's Defense drop after Stealth Rock damage, while unboosted Conkeldurr can 2HKO. As Cloyster is incredibly fast after Shell Smash, there aren't many Choice Scarf users that can outspeed it after it gets a boost. However, Latios can do so and will send the clam sinking back to the briny depths with Draco Meteor. Under the sun, Chlorophyll users, such as Venusaur and Sawsbuck, can opt to run Speed-boosting natures in order to be faster than a +2 Cloyster, as well.
Generally speaking, bulky Water-types and Steel-types can handle Cloyster. Bulky Rotom-W resists Cloyster's STAB moves and can take a Rock Blast. It can promptly OHKO Cloyster with Thunderbolt, making it a good switch-in to it in general. Jellicent and Tentacruel also have the capability to annoy Cloyster greatly. Both can use Scald on Cloyster and hope for a burn, among other options; Will-O-Wisp from Jellicent will help prevent Cloyster from being a huge problem, while Tentacruel can use Toxic, which will quickly wear Cloyster down when used in tandem with Protect. Slowbro is another bulky Water-type that can tank anything Cloyster can throw at it and target its weaker Special Defense stat with Psychic or Flamethrower; it can also use Thunder Wave on Cloyster to cripple it for the rest of the game. Keldeo deserves a special mention for being an offensive Water-type that can survive +2 Cloyster's offensive artillery with some health to spare.
Skarmory can't do much actual damage to Cloyster, but can Whirlwind it out to prevent the immediate problem, getting rid of Cloyster's stat boosts. Specially defensive Jirachi can quite easily take Cloyster's Water STAB move and retaliate with Thunder on a Calm Mind version or Body Slam and Iron Head on a physical version. Ferrothorn can take a +2 Icicle Spear and either hit back with Power Whip or cripple Cloyster with Thunder Wave. Thanks to Unaware ignoring Shell Smash boosts, Quagsire can take Icicle Spear nicely and Toxic Cloyster back, or even hope to burn it with Scald. A slightly unusual but effective option to deal with Cloyster is specially defensive Heatran; if Hydro Pump is favored over Razor Shell on Cloyster, it will be unable to OHKO Heatran even at +2. In return, Heatran will OHKO Cloyster with Lava Plume after Stealth Rock damage. Take note, however, that if rain is up, Heatran will be destroyed by any Water-type move from Cloyster.