Shell Shocked
Nature
Adament/Jolly
Ability
Skill Link
Moveset
Shell Break
Clamp/Razor Shell
Rock Blast
Icicle Spear/Explosion
Clamp gives you more power with skill link 175 POW to be exact, however, Razor Shell can lower defense and it's slightly more accurate, but I'd go with Clamp. Rock Blast gives you plenty of neutral coverage with the water attack. However, there are still dual types who can block this combination such as Water/Fighting, Water/Steel, Grass/Fighting, and Grass/Steel. And to make things worst each of those dual types is going to have at least 1 strong candidate for the OU tier (Keldeo, Empoleon, Breloom, Virizion, and Ferrothorn). The final 2 moves are both there for getting rid of threats Icicle Spear gets rid of the Grass/Fighting type and handles known threats like Garchomp, while Explosion gets rid of both Grass/Fighting and Water/Fighting as well as another other non ghost or steel.
Countering
Steel types give Cloyster so much trouble, notably Empoleon and Ferrothorn. Cloyster also has low HP and SD, and after a Shell Break it's DEF becomes more manageable, while it's bad SD becomes even worse, so Lucario and Toxicroak can easily revenge kill it with Vacuum Wave.
Nature
Adament/Jolly
Ability
Skill Link
Moveset
Shell Break
Clamp/Razor Shell
Rock Blast
Icicle Spear/Explosion
Clamp gives you more power with skill link 175 POW to be exact, however, Razor Shell can lower defense and it's slightly more accurate, but I'd go with Clamp. Rock Blast gives you plenty of neutral coverage with the water attack. However, there are still dual types who can block this combination such as Water/Fighting, Water/Steel, Grass/Fighting, and Grass/Steel. And to make things worst each of those dual types is going to have at least 1 strong candidate for the OU tier (Keldeo, Empoleon, Breloom, Virizion, and Ferrothorn). The final 2 moves are both there for getting rid of threats Icicle Spear gets rid of the Grass/Fighting type and handles known threats like Garchomp, while Explosion gets rid of both Grass/Fighting and Water/Fighting as well as another other non ghost or steel.
Countering
Steel types give Cloyster so much trouble, notably Empoleon and Ferrothorn. Cloyster also has low HP and SD, and after a Shell Break it's DEF becomes more manageable, while it's bad SD becomes even worse, so Lucario and Toxicroak can easily revenge kill it with Vacuum Wave.