Overview
########
Cloyster is one of few Pokemon lucky enough to be gifted with arguably the best boosting move in the game for a sweeper: in Shell Smash. In addition, Cloyster's ability, Skill Link, makes Cloyster's its Icicle Spear and Rock Blast essentially 125 Base Power moves. These two advantages mark Cloyster as one of the most threatening sweepers in the metagame. One thing often overlooked on Cloyster due to its sweeping role is its gargantuan Defense stat, which aids Cloyster it in finding opportunities to set up, as well as helps against physical priority. However, to compensate for its amazing Defense stat, Gamefreak decided to give Cloyster an abysmal Special Defense stat; this in conjunction with its poor defensive typing means that the majority of special moves will severely damage it and prevent it from being able to successfully pull off a sweep. Its Cloyster's typing also gives it a nasty weakness to Stealth Rock, as well as Mach Punch, a common priority move. In addition to its defensive weaknesses, Cloyster often has trouble getting past bulkier Pokemon even after it has set up, which makes it vulnerable to some of the more common threats in the metagame, such as Aegislash, Ferrothorn, Rotom-W, and Scizor. Nevertheless, despite Cloyster's flaws, it is still a deadly sweeper, and while it may take some skill to use effectively, it can be extremely dangerous in the right hands. [This kind of stuff isn't necessary to say, as it applies to literally anything and everything.]
Shell Smash
########
name: Shell Smash
move 1: Shell Smash
move 2: Icicle Spear
move 3: Rock Blast
move 4: Hydro Pump / Razor Shell
ability: Skill Link
item: Never-Melt Ice / Lum Berry
nature: Naive / Adamant
evs: 252 Atk / 4 Def / 252 Spe
Moves
========
This is Cloyster's most effective set, and is the set that gave Cloyster its reputation as an incredibly dangerous sweeper. Shell Smash is the move that makes Cloyster so threatening; by doubling its offensive stats and Speed, it Cloyster instantly becomes a threatening sweeper. Cloyster's main STAB attack, Icicle Spear, will consistently be 125 Base Power thanks to Skill Link, and after a Shell Smash, it will inflict massive damage on anything that doesn't resist it; even neutral [Sounds weird.] physical walls such as Ferrothorn and Skarmory take large amounts of damage. Rock Blast is an excellent coverage move, as not only is it . In addition to it being boosted by Skill Link, it also provides great neutral coverage, hitting Fire- and Ice-types super effectively, and is Cloyster's main way of dealing with Talonflame, Gyarados, Volcarona, and Mega Charizard X. The fourth slot is based on personal choice. Hydro Pump is often the better option, as it allows Cloyster to break through physically defensive checks such as Skamory, Forretress, and Scizor with a bit of prior damage, while Razor Shell is Cloyster's best option against Assault Vest Tyranitar, Specially Defensive Jirachi, and Specially Defensive Heatran. However, Ice Shard can be used to KO a weakened Breloom, hit Talonflame before it attempts to revenge kill with Brave Bird [I feel like this is something worth saying.], and Choice Scarf Noivern as well as giving Cloyster useful priority before it boosts, but it is often unnecessary.
Set Details
========
The EVs are allotted in the simplest way for a sweeper, with full investment in Attack and Speed to hit as hard and fast as possible. Naive is the preferred nature as it doesn't detract from Hydro Pump's power, but Adamant can be used with Razor Shell for maximum damage output. [What about Jolly?] There are many items viable on Cloyster, but generally Never-Melt Ice is the best option as it boosts the power of Cloyster's Icicle Spear without the debilitating recoil of Life Orb, specifically allowing Cloyster to beat Clefable, Conkeldurr, Genesect, Gastrodon, Mega Tyranitar, and Mega Venusaur after Stealth Rock. Lum Berry is a very helpful alternative as it offers protection from a status ailment which may be inflicted from a bulky opponent, such as Slowbro's Thunder Wave. King's Rock is a suitable item in conjunction with Skill Link as Cloyster's multi-hit moves gain a 41% flinch rate, though this is situational and usually an item that is always in effect is better. It does have some use in breaking down bulkier checks if the flinch chance goes your way, so it is definitely helpful in certain scenarios. White Herb negates the Shell Smash drops to better shield against priority once Cloyster is weakened, while Life Orb is an acceptable choice to increase Cloyster's damage output; it enables Cloyster to do a hefty amount of damage to Ferrothorn, Skarmory, and Rotom-W, but it decreases Cloyster's already short lifespan. Finally, Focus Sash can be used in order to guarantee a Shell Smash against powerful opponents, but with the ubiquity of entry hazards and the presence of sand, Cloyster rarely finds an opportunity to keep its Focus Sash intact. [You can probably mention that Focus Sash Cloyster is not that bad as a lead, as it can force a ton of offensive pressure right off the bat and weaken down defensive cores or even score a few kills against teams that lack an effective response.]
Usage Tips
========
Like all other sweepers, Cloyster cannot sweep before its counters have been removed. Take advantage of teammates such as Volcarona, Mandibuzz, and Latios; use them to eliminate troublesome Pokemon such as Aegislash, Scizor, Rotom-W, and Ferrothorn. Try to have as many entry hazards as possible on your opponents side, as this will aid Cloyster's sweep immensely. In order to set up, Cloyster should come in on something it forces out, or something that can't do much to it, and then use Shell Smash. Often it may seem tempting to lead with Cloyster and Shell Smash straight away, but try to avoid this is your opponent may have a counter that you are unprepared for. Also, with Cloyster's low Special Defense, it may not always find opportunities to set up, so make sure you take advantage of the moves your opponent makes to find a suitable moment. One thing to note is that because of the large threat Cloyster immediately poses, it often baits opponents to use an array of attacks from the get-go, and this can be used to your advantage by pivot switching into something that can take the hit and force the foe out, and then switching back to Cloyster.
Team Options
========
In terms of teammates, Cloyster mainly appreciates powerful special attackers that can take on the variety of foes that it can't itself. An excellent example of this would be Volcarona, who can take on the likes of Scizor, Breloom, Ferrothorn, Skarmory, and Aegislash before they have the chance to threaten Cloyster. Other Pokemon that perform similarly in this role include Latios with Hidden Power Fire, Rotom-H, and Heatran. Landorus-T and Mandibuzz can get rid of Cloyster's main counter, Aegislash, and Mandibuzz in particular can use Foul Play on many physical priority users, as well as Defog away entry hazards [You mention hazard removal support again later, so I don't know if you want to combine them somehow or just ditch this mention and tag it with the others.] . Cloyster always enjoys hazard support from the likes of Ferrothorn, Forretress, and Skarmory. Additionally, Cloyster enjoys hazard support on its own side as well, as it is weak to Stealth Rock and vulnerable to all other hazards; this can be in the form of either Rapid Spinners or Defoggers, and examples include Excadrill, Scizor, Latias, Starmie and Forretress (who can also set up hazards making it an excellent partner). Magnezone can trap and eliminate many of the Steel-types which threaten Cloyster, such as Scizor, Skarmory, and Ferrothorn. Cloyster also likes dual screen support as it gives it fantastic support while it is setting up. Aegislash can run Hidden Power Fire to act as a lure for the aforementioned Steel-types, as well as use the advantages of King's Shield to give Cloyster free switch-ins.
Other Options
########
Cloyster is one of few Pokemon with access to Rapid Spin as well as and Spikes, so it could attempt to pull off a defensive role, but it's Ice typing makes this a terrible idea. It can also run Explosion for when it is severely weakened, but it uses up a valuable moveslot and is generally a poor choice. Finally, a Choice Band or Choice Scarf set could be viable, but Cloyster has a noticeable lack in power if it uses Choice Scarf, or Speed if it uses Choice Band; therefore Shell Smash is almost always a better option.
Checks & Counters
########
**Aegislash**: Aegislash is the best counter to Cloyster, as it can take all of its moves in Shield Form and OHKO back with Shadow Ball in Blade Form due to Cloyster's horrible Special Defense; mono-physical variants can also retaliate with a combination of Sacred Sword and Shadow Sneak.
**Scizor**: Cloyster cannot OHKO Scizor with any of it's moves even after a Shell Smash, while Scizor can cleanly 2HKO with Bullet Punch or OHKO with Superpower.
**Mega Lucario**: Mega Lucario can switch in on any attack of Cloyster's bar Hydro Pump and Razor Shell at +2, and OHKO back with Vacuum Wave; due to this, Mega Lucario can also revenge kill Cloyster.
**Bulky Water-types**: Bulky Water-types are a very effective way of dealing with Cloyster as they resist both of its STAB moves. A common one is Rotom-W, who can take a hit from Cloyster and OHKO back with Thunderbolt. Quagsire also fills this role very well as it can take Cloyster's hits easily due to its Unaware ability and cripple Cloyster with Toxic, or try to burn it with Scald. Jellicent and Tentacruel perform very similarly as both can wear Cloyster down with Scald, and Jellicent can also Will-O-Wisp Cloyster, while Tentacruel can Toxic stall it out. Slowbro can take a hit from Cloyster and heavily dent it with Psychic or Flamethrower; it can also paralyze Cloyster with Thunder Wave, preventing it from successfully sweeping. Keldeo is not incredibly bulky, but it can survive any of Cloyster's attacks at +2 and retaliate with either Hydro Pump or Secret Sword. Bear in mind that Lum Berry variants of Cloyster are immune to an initial status move, so try to determine the item Cloyster is running from the damage output.
**Bulky Steel-types**: Skarmory can Whirlwind out Cloyster, eliminating its stat boosts, while Specially Defensive Jirachi can take all moves well except for a Razor Shell and either paralyze it with Thunder Wave or OHKO with Thunder. Ferrothorn is naturally bulky enough to take most hits and Power Whip or Thunder Wave in return.
**Mach Punch users**: Breloom and Conkeldurr can dent Cloyster heavily with Mach Punch, and are able to dispose of it with a bit of prior damage, but it must be noted that Breloom is heavily dented by the rare Ice Shard.
**Stealth Rock**: Cloyster's Ice typing gives it an unfortunate weakness to Stealth Rock, so having it up can limit Cloyster's switch-in opportunities and make it easier for opponents to defeat.
