Though rare, Empoleon is a hard counter to Cloyster, resisting pretty much every move Cloyster may carry and being able to OHKO with Grass Knot.
Damage calculations:
Empoleon @ Leftovers (0 EVs, Neutral Nature) Grass Knot vs Cloyster @ Life Orb (0/0 EVs, Neutral Nature): 121.9 - 144.3% (294 - 348 HP) OHKO
Cloyster @ Life Orb (252 EVs, Neutral Nature, +2) Rock Blast (5) vs Empoleon @ Leftovers (252/160 EVs, Neutral Nature): 35.7 - 42.2% (Total) (133 - 157 HP) 3HKO
Cloyster @ Life Orb (252 EVs, Neutral Nature, +2) Icicle Spear (5) vs Empoleon @ Leftovers (252/160 EVs, Neutral Nature): 26.6 - 31.4% (Total)(99 - 117 HP) 5HKO
Cloyster @ Life Orb (48 EVs, Neutral Nature, +2) Hydro Pump vs Empoleon @ Leftovers (252/92 EVs, +Nature): 34.1 - 40.3% (127 - 150 HP) 3HKO
Cloyster @ Life Orb (252 EVs, Neutral Nature, +2) Shell Blade vs Empoleon @ Leftovers (252/160 EVs, Neutral Nature): 32.2 - 37.9% (120 - 141 HP) 3HKO
Even a +2 Life Orbbed critical hit won't OHKO Empoleon, only way for Cloyster to get past is to carry King's Rock and hope he flinches Empoleon to death.