EEeEEEEEeeeeEEEEEeeEEEEEEEEEEE!
Shuckle I love you.
Anyway...these are the sets I run the best. Doubles is excellent for Shuckle because it allows him to get completely set up and ready in a short amount of time.
He pairs extremely well with Medicham and Smeargle, due to those two's ability to learn both Power Trick and Baton Pass. No coincidence, folks. Simply prepare a Telepathy Musharna to Trick Room and you've got a workable doubles (or even triples or rotation) team. Tyranitar and Hippowdon are a must-have for any Shuckle-blessed team because of their infinite Sandstorm.
Sturdy will protect Shuckle from OHKO's which, surprisingly, is useful if Shuckle is undamaged with Power Trick. Definitely an improvement over before.
Here's my singles set. I've found that Shuckle works well as an almost gimmicky last-man effort and it can actually take out any Pokemon ever provided it only has to take out one. Its not-to-be-trifled-with defenses, coupled with the moves below, are enough to defeat anything one on one.
As a side note, I have run this set in Ubers and believe me when I say that this team runs everyone else into the ground. Power Split cripples legendaries' attack and gives it to Shuckle, allowing the silly noodle rock to completely devastate. His movepool is godly for Sandstorm - if only he'd received Sand Force/Rush instead of some lame Contrary power. It's not even logical! Hydration, Magic Guard, Wonder Guard, or Sand [noun] would have been much better choices for abilities and they would totally have made sense, but instead...psh. Whatever. Maybe next gen they will see the error of their ways.
Shuckle @ [item] (I use either Mental Herb or Sitrus Berry personally, but almost any item will do.)
Sturdy/Gluttony (if you're using the Berry)
252 HP/252 Def/4 SpD (switch around or appropriate as you see fit. Make sure he keeps his HP high, though)
~Power Split
Interchangeable:
~Bug Bite
~Gyro Ball (place 0 IV's in speed for this)
~Earthquake
~Stone Edge
~Toxic
The objective here is not to cripple the foe so much as place Shuckle within an effective range for actually dealing damage. I find that this advantage disappears when a new foe is sent out so use Shuckle as a clean-up hitter instead of a driving force for your team.
Incidentally, giving Shuckle an evolution is the best thing Game Freak can do for it because Eviolite would make Shuckle shoot straight for Ubers.
Counters. Shuckle has no counters! No counters at all!
Just kidding. To counter a Shuckle, just use Magnezone. Suddenly, Shuckle is rendered completely and utterly ineffective for one round while it disables the Air Balloon, which is one round in which you can cripple it. While it is patently impossible to OHKO Shuckle even if Sturdy somehow didn't exist, a 2HKO is certainly possible, and a 3HKO is not ridiculous. Especially if you paralyze Shuckle and prevent it from moving. I find that Rock-types with huge offensive boosts and strong Rock moves are not conducive to survival in Shuckle terms. Other than that, just look for something that resists Shuckle's godawful movepool and use it.
(ogawdican'tbelieveI'madvisingpeopleaboutcounteringshuckle)
Pair Shuckle with things that can defeat its weaknesses (levitating steel primarily because of its resistance to everything Shuckle can use) and you should do fine. Works best against metagamers who can't understand why anyone would want to use Shuckle because they're addicted to Smogon.
EDIT: Reading through this thread makes me sad inside.
Come on, you guys. Shuckle is more than just a tool for lowering the opponent's offenses. While Knock Off, Gastro Acid, Toxic, and Power Split are really nifty as defensive tools, Shuckle has access to a few really great moves and can cover a wide variety of popular types
*Weavile turns in its sleep*
so there's really no need to senselessly hate on it. With the best defensive stats of all time, and with Power Trick the best offensive stats of all time, it is a force to be reckoned with.
Sure, I agree that it's Game Freak's superdefensive jokemon. Yes, I understand it seems about as useless as Magikarp...on the surface. If Magikarp can OHKO everything with some clever setup, think of what Shuckle could be capable!
...but I guess you're going to go back to your Choice Scarf Max Attack EV'd Swords Dance Sand Force Edge Quake Excadrill and wonder why you don't see too many Shuckles competitively.