Cloyster's purpose in OU revolves around one move—Spikes. Cloyster is one of only two OU Pokemon that can effectively lay Spikes, thereby dealing 12.5% damage per switch-in on all Pokemon, barring Flying-types. As a Spikes user, Cloyster boasts a decently powerful Surf and Explosion. These two moves in conjunction give it the ability to support its team offensively once Spikes are on the field. Cloyster also has decent Speed, which it can use to outrun and Explode on threats that its main competition, Forretress, cannot. Cloyster's better offensive synergy, STAB, and more dangerous Explosion are the primary reasons to use it over Forretress as a Spikes setter.
While Cloyster shines in its niche role as a user of both Spikes and Rapid Spin, it still has several glaring weaknesses. Cloyster's low HP and Special Defense, combined with a lack of reliable recovery outside of Leftovers, prevent it from taking more than a handful of attacks before being KOed. Forretress has more defensive staying power because of key resistances to Psychic- and Normal-type attacks, an immunity to Toxic, and only one weakness. Forretress has one other major selling point over Cloyster—Hidden Power Bug. Starmie is one of the most prominent spinners in OU, and with Hidden Power Bug Forretress can nail an important 2HKO after one round of Spikes damage. However, despite Cloyster's issues, it can still provide unique advantages as a Spikes user and should never be overlooked for a spot on either offensive or defensive teams.
This Cloyster is easy to use. Come in on a weak physical attacker or wall, set up Spikes, fire away with Surf or Toxic, and switch out when the opponent brings in one of Cloyster's counters. Spikes is an easy first choice and is the main draw to using Cloyster. The damage that Spikes deals per switch will punish opposing teams that try to double switch in order to gain better match-ups. Grounded prediction-oriented Pokemon such as Nidoking, Marowak, Machamp, and Rhydon will especially suffer from Spikes damage. Over a longer match, the damage from Spikes can play a key role in wearing the opponent down into KO range for boosting sweepers and will help grind down opposing stall teams. Surf provides an excellent offensive option for Cloyster to exercise against any Misdreavus, Gengar, Rhydon, or Skarmory hoping to switch into an Explosion. Surf is one of the main differences between Cloyster and Forretress. Unlike Forretress, Cloyster can go on the offensive against a wide range of opponents. Surf allows Cloyster to act as a solid Marowak counter on teams that lack Skarmory and as an additional support option against Snorlax. Cloyster also makes a pretty good Quagsire counter. Toxic is incredibly useful for hitting Water-type spinners such as Starmie and opposing Cloyster. Ice Beam can be used for super effective coverage against Zapdos and Exeggutor, but Toxic is generally a much better option. Explosion is the final option and gives Closyter an ace in the hole against Starmie, Raikou, Suicune, Snorlax, Blissey, and Zapdos. Explosion, used effectively, can help prevent Cloyster's team from being swept in emergency situations.Team Options & Additional Comments >>>
Cloyster is one of only four Pokemon that can use both Spikes and Rapid Spin. Combining the roles of both a Spikes user and spinner turns Cloyster into an effective utility Pokemon that can be used on both defensive and offensive teams. Spikes allows Cloyster to provide team support through passive damage. Rapid Spin allows Cloyster to clear Spikes from its own side of the field. With a combination of Rapid Spin and Spikes, Cloyster can fulfill two duties at the same time—clearing and laying entry hazards.
The third slot is for Surf as the obligatory STAB move. Surf provides a constant option to wear down opponents when necessary. Surf is suggested over Ice Beam because it does more damage against spinners such as opposing Cloyster and Forretress; it also offers better coverage against Ground- and Rock-types such as Tyranitar, Steelix, and Rhydon. The ability to go on the offensive with Surf, combined with Cloyster's better Speed, are the primary reasons for using Cloyster over Forretress as a defensive spinner and Spikes setter. The last slot is for a move of your choice. Toxic is the suggested option because it will give Cloyster an option against bulky Water-types that take little damage from Surf. Toxic will also force certain opponents to use Rest instead of attacking or to switch out to avoid more damage. Rest is useful to restore Cloyster's health, but without a cleric it's virtually useless. A sleeping Cloyster is nothing but dead weight and will often fail to survive the two turns required to wake up. However, Rest will allow Cloyster to become a 100% Steelix counter if there are no entry hazards on your side of the field. Ice Beam can be used if you desire super effective coverage against Dragonite, Exeggutor, and Zapdos, but Toxic is a far superior option overall. It is also good to keep in mind that Cloyster cannot use Explosion and Rapid Spin on the same set.Team Options & Additional Comments >>>
Beyond the stated moves, Cloyster has a few other interesting options it can use to surprise the opponent. Clamp is a good option when using Explosion because it will prevent the opponent from switching out to a Steel- or Ghost-type. Hidden Power Electric or Grass can give Cloyster an edge against opposing Cloyster in Rapid Spin and Spikes wars. Hidden Power will do more damage than Toxic right off the bat but it will fail to do significant damage to Starmie. Icy Wind is an option against faster opponents to lower their Speed. Icy Wind's Speed drop will give Cloyster an edge over Tentacruel, Raikou, and Starmie especially. The ability to lower the opponent's Speed can also boost Cloyster's chances of using Explosion before the opponent can set up Reflect, use Rapid Spin, or KO Cloyster. Reflect is another decent option for Cloyster. The extra Defense can let Cloyster survive a little bit longer and will give its team a nice boost when it switches out. However, it should be noted that like Explosion, Reflect is illegal with Rapid Spin.
Checks and Counters
Cloyster's entire purpose is to keep Spikes on the field; therefore, preventing this is the easiest way to check Cloyster. Rapid Spin users such as Starmie, Forretress, and opposing Cloyster make your own Cloyster's life a nightmare and decrease its ability to support its team. Starmie is not afraid of Cloyster's Surf and can easily switch in and remove Spikes with Rapid Spin. Opposing Cloyster, with their high Defense and resistance to Surf, are able to not only remove Spikes but also set up their own. Forretress is bulkier than both Starmie and Cloyster, is immune to Toxic, and resists Explosion. Like Cloyster, Forretress can both remove opposing Spikes and lay its own. However, because Cloyster's Surf will 3HKO, Forretress will not be able to take on Cloyster directly.
Beyond spinning, Cloyster is easily checked by powerful offensive opponents. While Cloyster boasts sky-high Defense, it has gutter-worthy Special Defense. Powerful Electric-types such as Raikou and Zapdos are especially threatening thanks to Thunderbolt's easy OHKO. Ghost-types, such as Gengar and Misdreavus, also make good checks because they have decent Special Defense, access to Electric-type moves, and the ability to block Rapid Spin and Explosion. Gengar will always OHKO with Thunder, and Misdreavus with Thunder can OHKO after one turn of Spikes damage. Cloyster, however, will seldom try to stay in on these opponents, preferring to use Surf or Toxic on the switch and then switching away to a fellow phazer or special wall. Powerful physical attacks will also spell doom for Cloyster. STAB Rock Slide will do a great deal of damage to Cloyster, but no Rock-types will enjoy switching in on Cloyster's STAB Surf. Keep in mind that using walls or tanks to check Cloyster is always a risky proposition because of the threat from Explosion and the possibility of Cloyster setting up Spikes or using Rapid Spin.