Slayer of Oysters Empoleon(Waterium Z)
Ability : Torrent
EVs : 252 HP / 228 SpA / 28 SpD
Yawn / Stealth Rock / Flash Cannon
Empoleon laughs at any attempt of retaliation from Cloyster so much that I am not even gonna put calcs here. From its side Empoleon easily destroys it. Hydro Pump can leave Cloyster with 1Hp(assuming Focus Sash) after Shell Smash and finish it with Aqua Jet if you want to save your Z-Move for another poke. The last slot can be arranged to whatever your team needs most.
The EVs on SpD are so Empoleon can always survive a Fire Blast from Timid Zard-Y and only 37% chance to get OHKO-ed from a modest one. Same statistics for Firium-Z Volcarona. Empoleon nails both after with Zard-Y always getting killed from Waterium Z plus Aqua Jet under drought.