Ice Shard Cloyster is quite rare, tbh, razor shell is far more used.
Anyways
Though it seems that cloyster after getting a shell smash boost he becomes unstoppable, everytime I used him, it's real role is to soften up opposing team, to preparare the sweep of another sweeper.
As offensive sinergy I find that Scizor is the best partner, by using hydro pump I can kill of easily Skarmory (91.01 - 107.18% at +2, so yeah, VERY high chance of ohko with sr) most of the physical wall that give Scizor problems are easily delt with cloyster. paving him an easy sweep, the only real problem for the ''core'' is the rare Quagsire, who nullifies boost completely, however, you could add a dd+sdhaxorus here, he has a nice defensive sinergy with scizor by being a dragon type and laughs at unaware