ive never used or seen a cloyster lead, but hereis my guess.
using surf, ice shard, rapid spin a fourth move (maybe toxic, ice beam or explosion) and focus sash cloyster is able to deal with all deoxys leads quite effectively. surf + rapid spin/ice shard is a pretty effective way to prevent your opponent from setting up SR + spikes from deoxys e/a leads, who account for over 1/4 of the leads.
also, if your opponent switches in giratina-o to spin block then you can hit it with a stab super effective attack...
i fail to see why this would be used over starmie however, as it can OHKO groudons that do not invest heavily in special defence with grass knot...