Clamperl

Shell Armor
Prevents critical hits.
Type Tier
Water UU
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

Sweeping Shell

DeepSeaTooth Quiet
Moveset IVs
~ Surf
~ Ice Beam
~ Hidden Power Electric / Hidden Power Grass
~ Confuse Ray / Brine
2 Spe
EVs
252 HP / 4 Def / 252 SpA

This set-up assumes you are using Trick Room on your team, because if you don't you will find Clamperl's shell crushed or fried before you can say "I pick you Clamperl!". That said, with DeepSeaTooth and max Special Attack we are talking about effectively 540 as a final Special Attack stat, which is downright amazing for a fully evolved Pokémon and just plain insane for something that has yet to evolve (except Pikachu).

So we have Surf as our main attack, which should do somewhere between 26% and 31% to Blissey if you ever have to face it. Use Ice Beam on Grass and Dragon Pokémon, and use the Hidden Power for other Waters. Hidden Power Grass is for Quagsire, while Hidden Power Electric is for Gyarados and Mantine. The last move is the epitome of a filler. Confuse Ray buys you a little time against something you can't OHKO, and Brine is better than Surf if your opponent is on lower than half health. Better as in can do 42% to Blissey-better.

The EVs, IVs and nature are tuned so that you are as slow as possible while still getting the right Hidden Power. You can get Hidden Power Electric with a Speed IV of 2 (not possible with 0). The minimum Speed IV for Hidden Power Grass is 3 (can only be with an odd number).

Other Options

Clamperl's movepool is terrible; there's nothing serious to find here. One could use Clamperl's other personal item (DeepSeaScale), which doubles its Special Defense, and try to tank with it. While 85 base Defense is decent for an unevolved Pokémon, 35 HP isn't going to help you tank anything except a L3 Spearow. If you're going to try this, make your way with Barrier, I guess. Toxic is an okay alternative for something like Ludicolo.

EVs

If you're using Trick Room, just use what's suggested. If you're not, use Modest and a 31 Speed IV and it will still work out fine. Clamperl's Speed is so low it won't outspeed anything significant even if you max it, so you're better off with maxing out that craptastic 35 HP and obviously his Special Attack.

Opinion

This thing should never be a serious consideration on any of your teams considering how frail it is. However, if you're sick to death of Pikachu, it might actually be fun to try this one on and actually hurt with it. 540 Special Attack is a really high stat after all. Once again, without Trick Room, using this thing is insanely hard.

Counters

The best counter to Clamperl is smacking it before it has a chance to attack you. If you are unable to OHKO Clamperl the moment it enters the battlefield, you're best off with the premiere special wall, Blissey. Ludicolo, Snorlax, and Lanturn can also take its hits decently, and hit back pretty hard. In UU play, Quagsire is your best bet if it doesn't have Hidden Power Grass, while Mantine is better if it lacks Hidden Power Electric. Hypno and Grumpig might stand a chance, though they can get 3HKOed by Surf.