Cloyster's ice typing screws him over though. Being pure water, Gorebyss doesn't have to worry so much about priority attacks, and she has HP Grass to take out other waters.
She plays an entirely different role anyway. While Cloyster is best as a physical sweeper, Gorebyss is best as a special sweeper. Cloyster COULD be a special sweeper, but Gorebyss has better sp atk for it. Her only handicap is that she lacks a priority move, but she becomes fast enough where it's usually not a big deal.
I don't see that many Brelooms anymore, and even if I am faced with a Breloom, I just bring out Shuckle to counter. I switch in, set up rocks, and as soon as Breloom attacks he's Red Carded out. And I don't think Gorebyss needs a Focus Sash, because she can usually survive an attack or two unless it's an attack from an offensive monster like Conkeldurr or Guts Raticate/Swellow.
I run Shell Smash, Surf, Ice Beam, HP Grass on Gorebyss. It gives her decent enough coverage, while Ice Beam takes out dragons (as well as flying types and most grass types) and HP Grass takes out bulky waters. As for Ferrothorn, he isn't a problem, because I can lead with a fire type, like Houndoom, which has not only good sp atk and decent speed, but access to strong fire type moves and taunt, and he can resist Power Whip like a boss to boot.