Most people I see use Cloyster as a lead to try and get in some damage. I'd lead with a non electric pokemon to carry thunderbolt, like Gardevoir. They'll try to use shell break, giving you the opportunity to take it out before it even gets a chance to hit you. You won't even need to try to outspeed it. I've yet to lose to a cloyster using this method. This works well with support pokemon that can be used as a tank, since they won't expect you to try to sweep with it.
If you want to wall it out, you could use a water/ice type like Dewgong, Walrein,or Lapras. If you want even more coverage, you could go with Empoleon. They have the bulk to take hits thanks to their resistances, and the lowered defenses from Cloyster can make it a lot easier to take him down.
If cloyster has a full speed of 262, then gets it doubled from Shell break, he'll have a speed stat of 564. Jolteon can have a speed of 394 at best. So you'd need an item to help him out if you wanted to use him. he would have a speed stat of 591.
