Cloyster

Water

Cloyster is an amazing setup sweeper and late-game cleaner due to its access to Shell Smash, one of the best setup moves in the game. Cloyster's Skill Link ability in conjunction with moves like Icicle Spear and Rock Blast allows it to have very high-Base Power moves while also breaking potential Focus Sashes on Pokemon such as Breloom. A sky-high Defense stat allows Cloyster to set up Shell Smash against most physical attackers with ease, and a good offensive Water / Ice typing gives Cloyster opportunities to sweep many types, including Grass, Ground, Dragon, and Flying, once its checks have been taken care of. However, a lack of coverage moves forces Cloyster to rely on the less powerful Hydro Pump to hit Steel-types. Cloyster also has relatively low offensive stats, which means it needs to set up to be an offensive threat, and its very low special bulk makes it take lots of damage from even weak special attacks. This, combined with a poor defensive typing with only two resistances, means Cloyster can have a hard time finding appropriate setup opportunities. This causes it to be extremely reliant on a Focus Sash to help it set up, and this reliance on Focus Sash also causes Cloyster to be very vulnerable to priority, which can revenge kill it easily. A mediocre Speed stat causes Cloyster to be outsped by most offensive threats, some such as Choice Scarf Latios and Swift Swim Kingdra even after a Shell Smash. Finally, it can be hard to justify Cloyster a teamslot when it is outclassed in most situations by other late-game cleaners and setup sweepers such as Mega Sharpedo, Gyarados, and Azumarill.

Ice

Cloyster is one of the most threatening Ice-type sweepers due to its Water typing and access to Shell Smash, which allows it to sweep types such as Fire and Rock if given the chance to set up. A neutrality to both Fire- and Steel-type moves allows Cloyster to heavily influence these matchups with Rock Blast and Hydro Vortex, being able to set up on Pokemon such as Infernape locked into Flare Blitz and Excadrill with ease. Furthermore, Cloyster's good offensive ability in Skill Link allows it to make good use of moves like Icicle Spear and Rock Blast, enabling it to break through Focus Sash from Pokemon such as Breloom, Alakazam, and Araquanid. A gargantuan Defense stat means it can set up in front of many physical attackers such as Excadrill and Mega Swampert easily. However, an abysmal Special Defense leaves Cloyster vulnerable to any decently strong special hit and diminishes its ability to set up as easily. Even resisted special hits, such as Scald from Keldeo, will dent Cloyster. Low offensive stats mean it is further dependent on a Shell Smash boost to pose as an offensive threat, since it doesn't hit very hard unboosted. Cloyster's mediocre Speed stat makes it somewhat easy to revenge kill, since it cannot outspeed key threats such as Choice Scarf Latios, and due to its vulnerability to entry hazards and Shell Smash's defense-lowering effect, it's easy to chip it down, possibly leaving it vulnerable to opposing priority moves such as Dragonite's Extreme Speed and Breloom's Mach Punch.