Barbaracle is pretty easily one of the most dangerous Pokemon introduced in Gen VI, even though it is not used frequently.
Barbaracle @White Herb/Mystic Water/Hard Stone/Muscle Band
Ability: Tough Claws.
152HP, 252 Attack, 104 Spe.
Stone Edge/Aerial Ace
This is my Barbaracle set. If you want to read why you should use it over Cloyster, read my argument in the two paragraphs below this one. Shell Smash is well you know Shell Smash; it doubles your Attack and Speed and is used for Sweeping, at the cost of Defense. The given EVs let you outspeed Base 130 after the Boost, with the rest in Attack and HP to make it easier to set up. It doesn't need a Focus Sash, and a Life orb will just make it die before it kills your opponent's team. Choice Scarf Users are a bit of concern; not so much Priority if you run White Herb. Razor Shell is Barbaricle's gold Standard Attacking move; it is accurate, boosted by Tough Claws, and its 50% Chance to make your Opponent's defense drop makes Barbaricle harder to counter, especially after a Shell Smash. Stone Edge is Barbaracle's Strongest STAB Rock Type move, but it's unfortunate that it is not boosted by Tough Claws. Cross Chop has great coverage with Stone Edge, and is boosted. Tough Claws Makes Aerial Ace viable at 80BP, which lets it hurt the Grass Types it despises harder than Stone Edge and Fighting Types that resist it's STAB such as Keldeo and Toxicroak, as well as Chesnaught and Breloom. Earthquake can be used for better accuracy over Cross Chop, but Cross Chop OHKO's Ferrothorn. Barbaracle has a very wide movepool, so pick anything that you think you might need.
When it comes to Shell Smashing, Cloyster is usually the first to come to mind. Barbaracle however, is by no means inferior, and is actually in many ways superior. Barbaracle's main disadvantage over Cloyster is that it lacks STAB Skill Link Icicle Spear, which does ~15% more Damage than Barbaracle's 100BP Razor Shell (Factoring Tough Claws), and lets Cloyster easily get past most Grass Types. But Barbaracle's most notable strength lies in it's coverage; it has practically anything for everything should it choose to run them, most of which that would otherwise be overlooked is made viable due to it's ability, Tough Claws. What can and can't stop Cloyster is definite; it doesn't have anything worthy of note to surprise it's opponents, and can always be dealt with by a single prepared team. When facing Barbaricle, you cannot be sure what will and won't be able to stop it; An Individual Team will never be able to stop every Barbaricle that comes their way, and that's what gives Barbaricle power. Furthermore, more directly speaking Stone Edge easily outdamages Rock Blast, and Cross Chop and Razor Shell outdamage Cloyster's Razor Shell. In other words, Icicle Spear is effectively the one and only thing Cloyster has offensively over Barbaracle. Defensively, however, Barbaracle is the easier of the two to set up, and that's another thing to consider!
Barbaracle has lower Defense, but just like Cloyster it is not going to drop to a Physical Attack easily. On the special side, however, Barbaracle has substantially more Sp. Defense than Cloyster. To get an idea, practically any Special Attack that Barbaracle is weak to will OHKO Cloyster even if hit neutrally. Barbaracle's Special Defense isn't even relatively bad, because of this, it isn't restricted to setting up on Physical Attackers without a Focus Sash; Yet another good reason to use Barbaracle. On top of not needing a Focus Sash, it can therefore run a different item, such as a boosting item to bridge the gap between that 15% difference of Razor Shell/Stone Edge over Icicle Spear. Life Orb is not recommended though. In terms of Defensive Typing, as I said earlier, it's Special Weaknesses are not a comparatively weak point because Cloyster will drop to the same attack when trying to setup. Barbaracle's Rock Typing, however, gives it a few more resistances to work with, most notably Fire and Flying. This means it can setup a Shell Smash on Talonflame, a very common Pokemon (Barbaracle can actually just barely do it without investment, let alone Cloyster) It's weakness to Earthquake hurts, but Cloyster is Weak to Rock, which is more readily influential. Finally, Barbaracle is easier to fit on weather teams; in rain, it has outright more offensive power, and in Sandstorm, it is immune to the residual Damage and gets a Sp. Defense boost, making it an obvious choice.