Can't honestly believe Kings Rock isn't already on site. I've been using that for a year, there have been multiple well-known and successful PO teams abusing it, etc etc. Anyway, like Gastrodon, Cloyster looks like a pretty bad choice for this project tbh, it just doesn't have options. Support sets are almost completely outclassed by Forry. The only thing I can think of apart from generic Shell Smash which actually works is:
Cloyster @ Choice Band
Jolly / Adamant, 4 HP / 252 Atk / 252 Spe
- Icicle Spear
- Rock Blast
- Ice Shard
- Explosion / Rapid Spin / Razor Shell
CB Oyster is actually pretty cool in the current metagame, where the likes of Lando-T and Hippowdon are extremely popular. Rather than struggling for most of a match to weaken Cloyster's counters, and then still having to find a setup opportunity for it, this Cloyster takes advantage of the turns it does get to do insane amounts of damage.
Icicle Spear is for spamming and is ridiculously strong, having a solid chance to 2HKO 252/0 Rotom-W after Stealth Rock. Rock Blast is for when you can afford to predict bulky water switch-ins. Ice Shard is really useful atm for picking off both Lando forms, as well as the omnipresent Dragons / Breloom / weakened mons in general. Finally, the last slot comes down to preference. I like Explosion for doing insane amounts of damage to the few physical walls you can't touch (Slowbro), but Razor Shell (for Rachi / Zong) or RS are also possible. This guy is just a really great wallbreaker in general for the likes of SD Scizor.