Cloyster is one of the most unique Water-types in the UU metagame due to his secondary Ice typing and large movepool. He is the only Pokemon in the UU metagame to carry all of Spikes, Toxic Spikes, Rapid Spin, and Explosion. These great support moves are backed up by an excellent Defense stat, although his HP and Special Defense are quite pathetic in comparison. While it sets him apart from other Water-types in the metagame, Cloyster's Ice typing is often more hurtful than helpful defensively. It gives him two more weaknesses, including one to Stealth Rock, and causes him to lose many resistances that a pure Water-type would otherwise boast. Offensively, Cloyster's STAB Ice Shard is quite useful for taking out faster, frailer Pokemon. Cloyster's decent offensive stats ensure that many Pokemon will have trouble trying to switch into his Surfs and Rock Blasts, especially frail Ghost-types trying to block Rapid Spin.
This set aims to stop the opponent from setting up while also setting up Spikes of its own. Rock Blast is Cloyster's most powerful attack (bar Explosion) and will do a lot of damage to anything that doesn't resist it. Ice Shard is there to finish off Pokemon who use a Focus Sash while also enabling Cloyster to function as an excellent check to various sweepers like Swords Dance Venusaur and Dragon Dance Altaria. Rapid Spin can be used over Explosion to get rid of the various entry hazards that have been set up on your side of the field. This means that Cloyster can set up his own Spikes and Rapid Spin the opponent's entry hazards away.
The EVs are specialized for the lead position because they are tailored to beat most of the common leads. The 160 HP and 96 Special Defense EVs allow Cloyster to survive Timid Alakazam's Psychic when it is not holding Choice Specs or Life Orb, and max Attack allows Cloyster to do major damage with Ice Shard, Rock Blast, and Explosion.Team Options & Additional Comments >>>
Cloyster is a great support Pokemon in UU and is a decent candidate for any team in need of Spikes or Rapid Spin support. Cloyster's main job is to set up Spikes, but he fulfills many other roles as well, unlike Omastar, who is slightly bulkier but nowhere near as versatile. In addition, Cloyster functions as a great Rapid Spin user because he can nail Ghost-types with a reliable attack, unlike Donphan or Hitmontop, who rely on Assurance and Foresight, respectively, to hit Ghost-types. Cloyster also has the ability to take a lot of physical punishment, but Slowbro's better typing means that Cloyster is generally outclassed in this role because Slowbro takes physical hits such as Choice Band Waterfall from Azumarill better than Cloyster. If your team has trouble with Pokemon such as Slowbro and Azumarill, Toxic Spikes works well over Explosion.
The advantages of Surf include hitting physically defensive Rotom and Spiritomb for decent damage, but Rock Blast will allow you to deal with Substitute users such as Alakazam and Mismagius more effectively, due to their lower Defense stat.Team Options & Additional Comments >>>
Cloyster can run a decent Choice Band set due to his usable Attack stat, but he is much better off being able to switch between attacks. Without the ability to switch attacks, Cloyster loses a lot of his versatility.
Cloyster also makes a decent wallbreaker with Surf, Rock Blast, Hidden Power Grass, and Ice Beam or Explosion. This will hit Pokemon such as Steelix and Donphan hard, while also nailing Chansey and Clefable. However, wallbreaking is better left to Pokemon such as Blaziken and Magmortar because they have much higher offensive stats and type coverage.
Checks and Counters
Cloyster is easily countered by defensively oriented Pokemon such as Milotic and Slowbro. However, any Pokemon switching into Cloyster should be wary of Explosion. Otherwise, it depends on Cloyster's attacking moves. Without Rock Blast, Pokemon such as Chansey and Clefable have an easy time switching in. If Cloyster lacks Surf, Rhyperior and Donphan can switch in. Hitmontop works as a counter to Cloyster as well because of his decent bulk and access to STAB Fighting-type attacks.