I would count on the nature as ivs and what EV’s you put on a Pokémon like cloyster I have my 1 as a timid set max speed and special Attack with zinc as the remaining 4evs with white herb as I am aware shell l smash lowers the defense and special defense stats
So this I recommend
Colyster
Nature timid
ability shell Armor
Item white herb
Move set
Ice beam, hydro pump aqua ring and shell smash