Do we have to wait for the mix and mega suspect on pheramosa to finish up before we can suspect anything? I am really getting tired of playing against shell smash because most of my teams have to run at least two different checks to it and they are still beaten, because there is no surefire way to beat multiple shell smash users.
Here are the various ways to fight shell smash and how they can be beaten:
1. prankster haze, but you probably lose the prankster hazer in a trade with the shell smash user. If they carry another one, which is often the case, you have lost now that your counter is dead. I've seen teams running 3 or even 4 shell smash users simply mowing other teams down.
2. Spectral thief doesn't work against normal types and since it doesn't have priority, your spectral thief user has to take a hit first, probably causing it to die.
3. Unaware users can easily be beaten by sunsteel strike/moongeist beam, and even if you use unaware Mega Gyarados which is fortunate enough to resist both of those, you can still be beaten if the shell smasher is a Dark-type Simple variant with a STAB 260 BP Power Trip. Yveltil is a really potent simple smasher thanks to its great bulk allowing it to take a single Extreme Speed from anything, and kill with a 260 BP Power Trip.
4. Topsy Turvy shuts down a Shell Smasher if you predict it right, since it now requires two turns to set up, but it doesn't work behind a substitute or against a Magic Bouncer, and if you run it with prankster, Dark-types are immune to it too, so usually it's ran without priority. So you still have to take a hit from one of three unknown coverage moves.
5. Priority is extremely unreliable against it because many smashers run dazzling. If they don't, they are probably running simple with a White Herb, which means that they don't even have to worry about defense drops, so they probably survive your priority and kill you anyway.
6. Imposters have to hope that they don't lose a speed tie. If they lose the speed tie, Shell Smash's defense drop ensures that the Imposter is at least severely damage, if not KOed. If the Shell Smash user also carries Spectral Thief, an Imposter must lose the speed tie in order to beat it, but it will still take huge damage if doing so. If the Shell Smasher is at full health and wears a focus sash, the Imposter almost always loses.
Shell Smash is so widespread that there are no true counters to it. Unskilled players can simply slap it onto a mon and totally turn their game around, even from a 5-2 clearly losing position. i've seen it happen. It constrains teambuilding because you have to run multiple checks to it and those checks can easily be beaten with a small amount of residual damage. For Example, Assault Vest Solgaleo takes 83-98% from a +2 Blue Flare from neutral natured Mewtwo-mega y. So it needs to be at full health to actually take that and Spectral Thief the boosts. Assault Vest Solgaleo is generally seen as a pretty solid counter to mewtwo Mega Y since it can take any move twice at +0, deal a lot of damage, and pivot back out, but all of that just goes away with Shell Smash. THe same thing can be said for things like Giratina countering Primal Groudon. It doesn't once Groudon has Shell Smashed, unless Giratina runs Spectral Thief which it probably does not, or is an Unaware variant. These can easily be lured. So there really are no counters to Shell Smash.
TLDR: Suspect test shell smash as soon as possible plz
You forgot about Sash/Scarf Imposter, Prank Dbond, and Mguard Sash Spectral Thief XP.
I personally run Prank Dbond Registeel, Scarf Imposter Blissey, and Unaware Yveltal, and those mons together usually soft checks Smashspam on their own. If that doesn't work, I always have my SashSmash Psysurge MMY and Pixilate Diancie-M in the back with Spectral Thief. Shell Smash is totally not centralizing, as you can tell XP.
(If you can't tell, I am 100% in agreement that Shell Smash is broken as fuk and needs to burn in Regular Hackmons with Chansey BOOMs, Sheddy's evil ability, THE POWR, Random 180+ base power moves (hmmm V-Create...), and Trapping abilities not named M-pull.)