Cloyster
Type Tier
Water / Ice BL
Statistics
Stat
HP
50
 303
Atk
95
 288
Def
180
 458
Spc
85
 268
Spe
70
 238
Name Moveset

Clamp

 ~ Clamp
~ Blizzard
~ Hyper Beam
~ Explosion

Clamp works like Wrap, except it sacrifices some accuracy (it is 75% accurate) for increased power and special STAB (Gengar doesn't screw it over like it does for Wrap). It cleans up paralyzed Pokémon really nicely, and slower ones like Chansey, Exeggutor, Snorlax, and Hypno. Additionally, if someone switches into Clamp and the attack connects, Cloyster can switch out next turn while the opposing Pokémon is unable to attack. It's definitely a great late-game (or even mid-game) lockout move.

Blizzard is his huge damage STAB (special) attack. Anything that Surf is effective against, so is Blizzard (in the standard metagame anyway). Furthermore, you only suffer a roughly 10% accuracy depletion in favor of an extra 25 base damage and a better chance to freeze-important if your opponent switches to Starmie to counter Cloyster. This is one of two moves Cloyster will use to finish off his opponent after Clamping them to within KO range. He won't get enough attacks off to worry about PP, so Ice Beam isn't necessary.

Hyper Beam is his other move. This takes care of things that Blizzard won't (or is at least more effective than Blizzard in some instances). For example, after Clamping a paralyzed Chansey, Alakazam, or Starmie, Hyper Beam would be the best choice to finish them off.

After Clamping a few Pokémon into watery graves, Cloyster can take down yet another with this self-sacrificing move. It's also good as a last resort, or as surprise tactic to get rid of something that could become a huge hurdle for your team (like a Starmie or Tauros). It could be a good strategy to blow up Cloyster when you are 2-1 at the end, as well.

Other Options

Use Double-Edge if you fear opponents being able to switch Gengar or a Rock into an Explosion (or Hyper Beam, in case of a Rock-type). Rest could work as well to keep Cloyster around even longer to make him more of a physical tank. Surf could also be possible if you want extra special damage (beware that you will have a hard time dealing with Chansey and such friends). Supersonic, Mimic, Substitute, and Reflect are also possible fun ideas to throw in on Cloyster should he have time to mess around.

Opinion

Cloyster can hold his own against Snorlax. He has increased in popularity as a Tauros counter considering his massive Defense (but lack of HP hurts him to an extent). Clamp, as all Wrap-like techniques, is a great asset, and Explosion is always good. Too bad all Cloyster usually does in standard is blow up on Starmie. Don't let this hold you off—Cloyster is great to toy with.

Counters

Starmie and Lapras are Water-types that resist Cloyster's Ice attacks and can then fire back with super effective Thunderbolts. Slowbro is another Water-type, and much more defensively oriented. He shakes off Cloyster's attacks very easily and can paralyze it or wear him down with Psychic. Victreebel and Venusaur can put a dent in him with critical hit Razor Leafs, but beware of the STAB Ice Beams/Blizzards. Jolteon can bang Cloyster hard with its Thunderbolts, as can all the Electrics; however, be careful switching in Zapdos since its Flying-type gives it an Ice weakness.