Based on a Lavos skeleton and fleshed out by me.
Cloyster is one of the most used Pokémon in OverUsed. It is one of only two OU Pokemon that can effectively lay Spikes. 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 a decent speed, which it can use to outrun and Explode on threats like Exeggutor, Vaporeon, Machamp, 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.
However Cloyster doesn't outclass Forretress in any case. It still has several glaring weaknesses. Cloyster's low HP and Special Defense, combined with a lack of 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 which is 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.
Offensive Spikes setter
Item : Leftovers
- Explosion / Rapid Spin
The main reason to use Cloyster is because this is the best spiker in the tier and can set up safely on most physical attackers. The combination of Surf + Toxic allows it to threaten all the rock and ground types as well as Snorlax or other things like Starmie coming in to spin. Send your Cloyster into 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. Explosion forces the opponent to be careful sending in Cloyster's checks such as Raikou or Zapdos. Cloyster can also learn Rapid Spin, it sacrifices the threat of boom for Spikes war advantage. However it is not possible to run both Explosion and Rapid Spin in the same moveset. Which means you have to do a choice on what is needed for your team.
Hidden Power Electric is almost a 2HKO on other Cloyster but means you can’t Toxic important stuff like Starmie and Snorlax. Ice Beam + HP Elec counters other offensive teams but leaves Cloyster very vulnerable to Forretress. HP Fire is an option to nail Forretress but it lowers the HP DVs significantly enough that an observant opponent can notice the set before it’s revealed. Screech + Clamp is a rare but potent combo that can target Snorlax attempting to Curse in Cloy’s face.
[CHECKS & COUNTERS]
Almost everything that outspeeds Cloyster can threaten it, prime examples being Zapdos and Raikou which OHKO with a STAB move. Cloyster can’t kill Starmie one-on-one without using Explosion, and Starmie outspeeds and spins, so they often end up trading. Cloyster is vulnerable to status, paralysis or sleeping can be crippling as it prevents the relatively fast Explosion which is what makes Cloyster really shine in most games. Forretress loses to Cloyster but can still chip it down with Giga Drain, and it can run Spikes + Rapid Spin unlike Cloyster which means dealing with it should be a priority.