I actually agree that Shuckle should probably be suspected. No matter how passive it is, you can't kill the thing. A +2 dragonite outrage (default base attack) can't even 2hko it through leftovers recovery! (I should have saved more replays but this is the only one I have)
http://replay.pokemonshowdown.com/giftsofthegods-450025426
Plus it's only hit super effectively by rock, steel and water-type hits. With contrary shell smash it becomes even tougher to take down to the point that the only common way to destroy it is with critical hit, status and/or phasing. The less-common way to halfway disable the thing is with trick, but even then it's so bulky it's guaranteed to set up sticky web and toxic a few people before it goes down. I don't run shell smash myself, since I have to give up something else I like, but I've seen what it can do. The set I use is:
Impenetrable (Shuckle) @ Leftovers
Ability: Sturdy
EVs: 252 HP / 4 Def / 252 SpD
Careful Nature
- Sticky Web
- Rest
- Knock Off
- Toxic
Knock off is great, and I haven't seen it mentioned at least in the last 30 posts. I should actually replace sticky with contrary since nothing can ohko shuckle fro full health. Taking out leftovers recovery is really something that offensive mons love.
OK, so you can shut it down with taunt or trick, you can stop it from statusing you or knocking off your items with sub. The real problem with Shuckle is that it's so centralizing and there's like 0 opportunity cost in using it over some other bulky mon. A team with a shuckle will outperform the same team with another mon in its place sinmply because Shuckle can outlast so many things, spread status, or put enormous pressure on hazard removers. I think the only defogger that can deal with Shuckle is Scizor and the only spinner is Excadrill and both of those aren't really that common in this meta. I guess a sand-rush Drill gifted with 180 atk might be useful though (hmm).
In short, Shuckle is great for pressuring almost all defensive mons and paving the way for a sweep from one of your offensive mons. It fits great even on HO teams because of this. Indeed, Shuckle's bulk is so good that you don't really have to worry about losing momentum since Shuckle's almost guaranteed to stick around for a long, long time.