Alright first off why do you have a focus sash with rapid spin? Coming into rocks to spin but making your sash useless now...AND a weakness to rocks..not a good idea. Also, Pokemon who are weak to ice generally dont do its job the best with spinning. Here, Ill suggest this moveset for you
Even though he boosts with shell smash, he still gets stopped by very defensive pokes especially steels if you have razor shell. Especially metagross since he doesnt get his defense lowered by razor shell due to clear body. And since you have a sash, the spdef drop in naughty nature wouldnt matter and besides his spdef is just too horrible to begin with so a decresing nature wont hinder it in anyway. So mixed cloyster with hydro pump is best for those physically bulky pokes. The only problem is priority moves so make sure to take care of them before cloyster attempts his sweeps. And make sure you have a different spinner (not him) so he can come in late game and not have his sash ruined.