Cloyster

Cloyster has a solid niche in ADV OU because it is a Spiker with unique traits. Being a Water-type means that it naturally matches up well against opposing Water-types by freely setting Spikes against the likes of Swampert, Suicune, and Milotic, and not being a Steel-type means that it isn't trapped by Magneton unlike Skarmory and Forretress. However, it misses out on the immunities to sand and Toxic that the Steel typing provides and the many useful resistances to common attacking types.

Compared to the other Spikers in the tier, Cloyster is not only a better choice against Magneton but also a much more threatening attacker with its good Speed combined with the coverage of its STAB Water- and Ice-type attacks that can threaten most foes. Notably, it threatens a very common spinner in Claydol but fails to threaten another common one in Starmie. Additionally, its Explosion can be used to create a hole in the enemy team by severely damaging a wall, preventing the opponent from setting up with their sweeper, or ending the turn and giving your own sweeper a free opportunity to set up.

Despite all this, Cloyster has many shortcomings that hold it back from seeing more usage as a Spiker in OU. First, it struggles to make progress against Rapid Spin Starmie; second, its Water / Ice typing sports many weaknesses to common attacking types such as Rock, Electric, and Grass and, paired with its low Special Defense, means it is threatened by the majority of the tier; third, it's easily worn down by residual damage because it lacks access to a recovery move and is weak to sand, Spikes, and Toxic, which is why it isn't seen much on teams that want to simply outlast the opposition.