Counter that Pokémon!

[Abby]

Bastiodon @Chesto Berry
Ability: Sturdy
EVs: 252 HP 200 SpD 56 Def
Careful Nature
-Taunt
-Rest
-Toxic
-Protect


Bastiodon can taunt, to stop the use of three of clefable's moves, and whittle it down with toxic, resting if needed while taking moonblasts

 

[Frozocrone]

It's like Celesteela can counter most mons, just ban it already xD

I'm going to go with another option though:

Nidoking @ Life Orb
Ability: Sheer Force
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Earth Power
- Sludge Wave
- Ice Beam
- Flamethrower

Earthpower 3HKO or 2HKOs depending on whether Bastiodon uses Rest. Bastiodon can't touch Nidoking in return.

 

[gimmepie]

Chansey @Eviolite
Ability: Natural Cure
EVs: 4 HP / 252 Def / 252 Sp. Def
Bold Nature
-Toxic
-Soft Boiled
-Seismic Toss
-Heal Bell

Chansey has exactly nothing to fear from Nidoking, being able to easily remove any status affliction it cops as well as heal the already negligible damage Nidoking can inflict all the while whittling Nidoking down with Seismic Toss.

 

[Zeffy]

Breloom @ Toxic Orb
Ability: Poison Heal
EVs: 12 HP / 244 Atk / 252 Spe
Jolly nature
- Swords Dance
- Mach Punch
- Drain Punch
- Seed Bomb

go away chansey no one loves you

 

[gimmepie]

Talonflame
Ability: Gale Wings
EVs: 252 Atk / 4 Sp. Def / 252 Spe
Adamant Nature
-Acrobatics
-Flare Blitz
-Roost
-U-turn

I think this one is self-explanatory.

 

[Frozocrone]

Tyranitar @ Choice Scarf
Ability: Sand Stream
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Stone Edge
- Pursuit
- Superpower
- Crunch

U-turn does around 1/3. Sandstream breaks Gale Wings Roost.

I'm not too sure, but I think Stone Edge might OHKO.

 

[gimmepie]

Tangrowth @Assault Vest
Ability: Regenerator
EVs: 252 HP / 4 Def / 252 Sp Def
Sassy Nature
-Leaf Storm
-Knock Off
-Earthquake
-Hidden Power Ice

TTar can 3HKO with Sandstorm damage at best but is OHKOd by Leaf Storm. Tangrowth can then simply switch out to recover any damage taken.

 

[5qwerty]

Tangrowth @Assault Vest
Ability: Regenerator
EVs: 252 HP / 4 Def / 252 Sp Def
Sassy Nature
-Leaf Storm
-Knock Off
-Earthquake
-Hidden Power Ice

TTar can 3HKO with Sandstorm damage at best but is OHKOd by Leaf Storm. Tangrowth can then simply switch out to recover any damage taken.

Sorry to be that guy, but...

Why was this the set instead of the Rocky Helmet one? Your set gets 3HKO'd by Stone Edge (69% chance by Crunch after Sandstorm damage) and can't actually OHKO in Sand because Rock-types get the SpDef boost. Here is calc:
0 SpA Tangrowth Leaf Storm vs. 0 HP / 0 SpD Tyranitar: 204-240 (59.8 - 70.3%) -- guaranteed 2HKO (so this factors in Sand because TTar is assumed to have Sand)

Essentially what would happen is, you come in on a Stone Edge/Crunch, take a Stone Edge/Crunch as you Leaf Storm, and then die to Stone Edge (possibly living Crunch) and then you KO with EQ (assuming you live). I guess you could switch out before after Leaf Storm, but I wouldn't really call it a counter.

EDIT: Oops I'm supposed to post an appropriate counter.

Landorus-Therian @ Choice Scarf
Ability: Intimidate
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Earthquake
- U-turn
- Stone Edge
- Knock Off

TTar Stone Edge 3HKOs at best after Intimidate, while Lando definitely 2HKOs with EQ (and is faster).

 

[gimmepie]

Sorry to be that guy, but...

Why was this the set instead of the Rocky Helmet one? Your set gets 3HKO'd by Stone Edge (69% chance by Crunch after Sandstorm damage) and can't actually OHKO in Sand because Rock-types get the SpDef boost. Here is calc:
0 SpA Tangrowth Leaf Storm vs. 0 HP / 0 SpD Tyranitar: 204-240 (59.8 - 70.3%) -- guaranteed 2HKO (so this factors in Sand because TTar is assumed to have Sand)

Essentially what would happen is, you come in on a Stone Edge/Crunch, take a Stone Edge/Crunch as you Leaf Storm, and then die to Stone Edge (possibly living Crunch) and then you KO with EQ (assuming you live). I guess you could switch out before after Leaf Storm, but I wouldn't really call it a counter.

EDIT: Oops I'm supposed to post an appropriate counter.

Landorus-Therian @ Choice Scarf
Ability: Intimidate
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Earthquake
- U-turn
- Stone Edge
- Knock Off

TTar Stone Edge 3HKOs at best after Intimidate, while Lando definitely 2HKOs with EQ (and is faster).

You got me on that one, I did that off the top of my head instead of checking the calc like I normally do. Here's a Lando counter

Chesnaught @Leftovers
Ability: Bulletproof
EVs: 252 HP / 252 Def / 4 Sp. Def
Impish Nature
-Leech Seed
-Synthesis
-Drain Punch
-Spiky Shield

The best Landorus can do is 9HKO with EQ after Leftovers. Drain Punch is only a 9HKO itself however Leech Seed, Spiky Shield and Synthesis make this a moot point. Lando is pretty much forced to switch or die a slow death.

 

[Nah]

Tornadus-Therian @ Assault Vest
Ability: Regenerator
EVs: 96 HP / 160 SpA / 252 Spe
Hasty Nature
-Hurricane
-Knock Off
-U-Turn
-Superpower

Given that it takes only one Hurricane to annihilate Chesnaught, its healing ability is rendered moot, and Drain Punch does at best 21%.

 

[InvisibleWater]

Tapu Koko @ Life Orb
Ability: Electric Surge
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Thunderbolt
- Dazzling Gleam
- Hidden Power Ice
- U-Turn

Tapu Koko resists all of Tornadus-Therian's moves, allowing it to switch in safely and OHKO Tornadus with Thunderbolt (hazard damage may be required if it switches in on a Knock Off, though)

 

[Frozocrone]

Marowak - A @ Thick Club
Ability: Lightening Rod
EVs: 248 HP / 252 Atk / 8 Spe
Adamant Nature
- Bonemerang
- Flare Blitz
- Shadow Bone
- Substitute

Dazzling Gleam is a 4HKO. Aim for the horn with Thunderbolt.

Bonemerang/Flare Blitz/Shadow Bone all OHKO (Shadow Bone has 37.5% chance).

 

[gimmepie]

Pelipper @Damp Rock
Ability: Drizzle
EVs: 248 HP / 252 Def / 8 Sp Atk
Bold Nature
-Roost
-Hydro Pump
-Hurricane
-U-Turn

Flare Blitz 5HKOs at best, Bonemerang does literally nothing and whilst Shadow Bone has a good chance to 2HKO, Pelipper outspeeds even without Speed investment to OHKO with Hydro Pump and can Roost that damage off.

 

[Zeffy]

Zapdos @ Leftovers
Ability: Pressure
EVs: 252 HP / 60 Def / 180 SpD / 16 Spe
Calm nature
- Thunderbolt
- Heat Wave
- Hidden Power Ice
- Roost

this should be good, rain boosted scald is as a 4hko at best while thunderbolt is an ohko

 

[InvisibleWater]

Kyurem-Black @ Life Orb
Ability: Teravolt
EVs: 4 Atk / 252 SpA / 252 Spe
Mild Nature
- Ice Beam
- Earth Power
- Fusion Bolt
- Roost

Kyurem-B resists Thunderbolt and HP Ice, and is only 4HKOed by Heat Wave, allowing it to safely switch in and OHKO Zapdos with Ice Beam after one round of hazard damage.

 

[gimmepie]

Conkeldurr @Assault Vest
Ability: Guts
EVs: 252 Atk / 236 Sp. Def / 20 Spe
Adamant Nature
-Drain Punch
-Mach Punch
-Knock Off
-Ice Punch

Kyurem 3HKOs at best whilst Conk can OHKO or 2HKO with Drain Punch and heal back damage taken simultaneously. Mach Punch also 2HKOs.

 

[Nah]

Azumarill @ Choice Band
Ability: Huge Power
EVs: 172 HP / 252 Atk / 84 Spe
Adamant Nature
-Play Rough
-Waterfall
-Aqua Jet
-Knock Off

Resists Conk's entire moveset, and even if he Band gets knocked off Play Rough still OHKOs:
252+ Atk Huge Power Azumarill Play Rough vs. 0 HP / 0 Def Conkeldurr: 372-440 (105.9 - 125.3%) -- guaranteed OHKO

 

[gimmepie]

Venusaur @Venusaurite
Ability: Overgrow (Will be Thick Fat)
EVs: 248 HP / 96 Def / 144 Sp. Def / 20 Spe
Bold Nature
-Giga Drain
-Sludge Bomb
-Synthesis
-HP Fire

Azumarill 4HKOs at best whilst M-Venu can 2HKO off of Sludge Bomb or Giga Drain, also healing itself off of the latter. Knock Off doesn't work on Mega Stones.

 

[PlatinumDude]

This is a bit unorthodox, but here goes:

Necrozma:

-Calm Mind
-Stored Power
-Iron Defense
-Moonlight
Nature: Bold
EVs: 252 HP/232 Def/24 Spe
Item: Leftovers

Defensive Mega Venusaur's attacks 3HKO Necrozma at best; from there, Necrozma can set up Calm Minds (and Iron Defenses), then take out Venusaur with Stored Power after two Calm Minds:

+2 0 SpA Necrozma Stored Power (100 BP) vs. 248 HP / 144 SpD Venusaur-Mega: 402-474 (110.7 - 130.5%) -- guaranteed OHKO

 

[Frozocrone]

->

Houndoom @ Houndoominite
Ability: Flash Fire
EVs: 40 HP / 8 Def / 208 SpA / 252 Spe
Timid Nature
- Nasty Plot
- Fire Blast
- Dark Pulse
- Will-O-Wisp

Yes, hello, I counter Necrozma by setting up alongside.

I was tempted to call out Necrozma against Venusaur, since Sludge Bomb 3HKO (even after switching in and Calm Minding), but the win could be pulled out via Moonlight on the first turn and then setting up.