Clamperl

Shell Armor
Prevents Critical Hits.
Type Tier
Water NFE
Statistics
Min- Min Max Max+
HP
35
- 211 274 -
Atk
64
147 164 227 249
Def
85
185 206 269 295
SpA
74
165 184 247 271
SpD
55
131 146 209 229
Spe
32
90 100 163 179
Name Item Nature

Special Sweeper

Deepseatooth Modest
Moveset EVs
~ Surf
~ Ice Beam
~ Hidden Power Electric / Hidden Power Grass
~ Toxic / Substitute / Confuse Ray
252 HP / 252 SpA / 4 Spe

Basically you have 540 Special Attack, 35 base HP and are slower than Wobbuffet. Send Clamperl in something that can't OHKO it and won't attack you on the switch, such as Skarmory or a sleeping Pokémon, Surf and Ice Beam everything in sight, switch to the Hidden Powers when Gyarados or Swampert (or even Omastar, assuming you're using this on underused play) pops up. If you're not good at predicting, Substitute will help a little, emphasis on a little considering Clamperl's HP and lack of Leftovers recovery. Confuse Ray might help you get in a hit or two, Toxic may help because Clamperl has to switch in and out constantly, though it won't do that good against defensive waters compared to other offensive waters such as Gorebyss and Omastar, because the Hidden Powers will HURT, a CH Hidden Power Grass/Electric from this thing OHKO's Suicune for example.

Name Item Nature

Staller

Deepseascale Bold
Moveset EVs
~ Barrier
~ Rest
~ Sleep Talk / Confuse Ray / Surf
~ Toxic
252 HP / 4 Def / 252 SpD

Novelty, but Clamperl itself is novelty anyway so worth mentioning. It would be decent if not for the 35 base HP and lack of Leftovers hurts a lot, but it's quite fun if you can pull it off.

EVs

On the first set, max Special Attack and Modest, that's for sure. Then you have some options, you can give it Defense, take a Dragon Danced Salamence Hidden Power Flying (or one of a Choice Banded one that switches in to take your Surf and ruin Clamperl and the rest of your team) and Ice Beam its face back, go with SDefense and spam "jajaja" when Omastar attempts to HP Electric/Grass you and it fails to OHKO, or you can give it Speed to outspeed something like the Regi Trio and Swampert. On the second, max HP and Special Defense with a Bold nature would do fine.

Opinion

Leave the clams alone, or even, feed it to your Grumpig or something, maybe it'll decide to grow another pearl and double its SAttack, now THAT would be cool. But Clamperl is still pretty cool, if you're looking for a change on your UU team, go for it. 540 SAttack is nothing to mess with, and UUs don't have good Defenses to begin with. Beware, though, UU metagame is all about offense, so don't get your clam get killed in the process.

Counters

Blissey, Snorlax, Lapras and Lanturn are the obvious ones. Other than that, you need to predict and switch something that can KO it. And nearly everything can KO it, as it's really slow and has no Leftovers recovery, so what it takes, it keeps. It can't switch in anything, as well.