Here is a set that people seem to run that just kills the entire meta, and this is why Shell Smash needs to be banned. There is very limited counterplay to this thing. The set is:
Mewtwo-mega-x @ Focus Sash / Safety Goggles
Ability: Dazzling / Psychic Surge / Illusion
EVS: everything
Whatever nature you want as long as it doesn't lower your offensive stats
- Shell Smash
- Close Combat / Low Kick
- Moongeist beam / Sunsteel Strike
- Stored Power
You need both Close Combat and Stored power as STAB moves. Stored power becomes a 140 BP STAB move coming off a very respectable 154 Special Attack after Shell Smash, and if you have Psychic Surge support, it becomes an efective 260 BP STAB move instead. That hits any neutral mon exceedingly hard. For resists, you have Close Combat, which also blasts through Steel types, so that means, at best you sac something, then bring in your Steel type, and Destiny Bond the Mewtwo, losing two mons for the price of just one. Finally Moongeist Beam/sunsteel takes care of Unaware Pokemon. To hit unaware Fairy types use Sunsteel, and to hit Shedinja use Moongeist beam.
What can counter this thing?
Nothing. You only have checks, which require a sac, and the mewtwo user can easily play around those. Here are some calculations:
Let's start with some common bulky walls:
+2 252 Atk Mewtwo-Mega-X Close Combat vs. 252 HP / 252 Def Solgaleo: 394-465 (82.4 - 97.2%) -- guaranteed 2HKO
+2 252 SpA Mewtwo-Mega-X Moongeist Beam vs. 252 HP / 252+ SpD Solgaleo: 384-452 (80.3 - 94.5%) -- guaranteed 2HKO
+2 252 Atk Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Fur Coat Registeel: 284-336 (78 - 92.3%) -- guaranteed 2HKO after Leftovers recovery
+2 252 SpA Mewtwo-Mega-X Moongeist Beam vs. 252 HP / 252+ SpD Assault Vest Aegislash-Shield: 180-212 (55.5 - 65.4%) -- guaranteed 2HKO (Yes, AV Aegislash can live a stored power followed by a Moongeist beam, but it can't take two, so with good rediction it will die. And AV Aegislash is such an uncommon set compared to the prankstervariant, you're wasting Aegislash's value as a Prankster user this way.)
Kyogre-p can be 2hKOED no matter what set it's carrying or what STAB move Mewtwo decides to use. In the worst case situations, you have
+2 252 Atk Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Fur Coat Kyogre-Primal: 202-238 (50 - 58.9%) -- 73.4% chance to 2HKO after Leftovers recovery
+2 252 SpA Mewtwo-Mega-X Stored Power (140 BP) vs. 252 HP / 252+ SpD Assault Vest Kyogre-Primal: 178-210 (44 - 51.9%) -- 12.5% chance to 2HKO It is ohkoed by close combat though)
+2 252 Atk Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Eviolite Fur Coat Chansey: 692-816 (98.2 - 115.9%) -- 87.5% chance to OHKO
+2 252 Atk Mewtwo-Mega-X Sunsteel Strike vs. 252 HP / 252 Def Sableye-Mega: 197-232 (64.8 - 76.3%) -- guaranteed 2HKO
+2 252 SpA Mewtwo-Mega-X Moongeist Beam vs. 252 HP / 252 SpD Sableye-Mega: 177-209 (58.2 - 68.7%) -- guaranteed 2HKO
+2 252 SpA Mewtwo-Mega-X Moongeist Beam vs. 252 HP / 252+ SpD Assault Vest Deoxys-Defense: 170-202 (55.9 - 66.4%) -- guaranteed 2HKO
That's just a sample of the destructive power this thing has. Note that the stored power calculations don't have psychic surge support, allowing Mewtwo to not reveal its ability. With Psychic surge support such as from a MMY partner, you don't have to bother predicting because you can just spam your Stored Power. Also remember that it outspeeds EVERYTHING that hasn't set up, even the entire Scarf metagame. So really you would need something like Focus Sash + Shell Smash MMY to offensively check it, but you're still sacrificing a mon to do so. If you decide to run Illusion instead, you can often take out an extra mon, such as a Steel-type, in a single hit, easily paving the way for an -ate user, and then bringing it in late game to finish off with a Shell Smash sweep. However, that leaves you vulnerable to priority moves, such as Pixelate or Aerilate Extreme Speed, which does OHKO you.
Also, this thing has a good chance to KO its own imposter with Moongeist Beam if it carries a negative spd nature, 0 IVS and 0 evs in SpD, which really isn't much of a tradeoff given the fact that you can't really take hits afte a smash anyway (+2 252 SpA Mewtwo-Mega-X Stored Power (140 BP) vs. -1 252 HP / 0- SpD Eviolite Mewtwo-Mega-X: 669-787 (95 - 111.7%) -- 68.8% chance to OHKO).
But really, if the best counter you can come up with is a failed prediction against Assault Vest Aegislash, that is really saying something, and if the best check is to lose two mons just to take it down either through a sac into destiny bond or a sac into Focus Sash, there is something wrong here.
We can argue that a few different things make this set broken. The most obvious problem is Shel Smash, which gives Stored power such high BP, DQM which gives it immunity to being revenged by priority moves, or psychic Surge which protects it from priority as well as powering up the stored power. I argue that Shell Smash is the biggest problem with it though. Any ofensive mon can run it, and I have already mentioned this before. But this is just ridiculous. THERE ARE NO COUNTERS TO THIS SET.
Finally, did I mention, this is a SINGLE mon running this? That gives you five extra teamslots that you can do whatever you want with. It doesn't really require any support. That is just wild to me. Please, do something!
TLDR: ban SHELL SMASH
Edits and additions:
Thanks to the people on Discord, there are more nuanced parts of the argument to discuss. For one thing, there is a wall that can take hits from this set pretty easily, and that is Unaware Zygarde-C, which Moongeist and Sunsteel both fail to 2hKO. That's at least something good, and fairly significant since unaware Zygarde-C is a very valid set to run.
Another argument they are bringing up is the one about imposterproofing, which you can do with the afformentioned Unaware Zygarde-C. For more offensive teams, you can run Prankster Aegislash if you opt to run Sunsteel Strike over Moongeist Beam on your MMX, which is probably the better move since it allows you to blast through unaware Fairies.
Finally, and the biggest counterargument, is that they argue that it is difficult to get Mewtwo set up in the first place. I'm inclined to disagree with this though, since you cannot passively click some moves if it invites a free MMX switchin, and then the arms race(?) of underspeeding threats to get slow u-turns for your defensive mons continues. U-turning against MMX does break the sash, allowing your Imposter to probably beat it one on one even if you minimize the Special defense Evs and Ivs, which is also a little better, but again, we are talking in a perfect ideal scenario where you haven't taken any chip damage on any of the relevant mons, which is not at all true in a real game. If your imposter takes even 13% and hasn't recovered that damage off, it has to win the speed tie, andsince IMposter works as a sort of team glue, it's very likely that it will hvae taken some prior damage.