Counter that Pokemon! (READ THE FREAKING FIRST POST)

[Pendraflare]

Latios (M) @ Choice Scarf
Ability: Levitate
EVs: 252 SpA / 4 SpD / 252 Spe
- Draco Meteor
- Hidden Power Fire
- Psyshock
- Trick

Keldeo does nothing to Latios with either of its STABs, while it gets slammed thanks to Psyshock hitting its lower Defense stat even if it uses Calm Mind.

 

[Nah]

Conkeldurr w/Assault Vest
Nature: Adamant
Ability: Guts
EVs: 252 Attack, 252 Sp.Defense, 4 Defense
Moves: Drain Punch, Mach Punch, Knock Off, Ice Punch

 

[Master Kavar]

Pidgeot@ pidgeotite
Ability : early bird
Nature: mild
Evs spd 252 atk 252 sp. Def4
Attacks
Heat wave
Hurricane
Roost
Tailwind

 

[PlatinumDude]

Pidgeot@ pidgeotite
Ability : early bird
Nature: mild
Evs spd 252 atk 252 sp. Def4
Attacks
Heat wave
Hurricane
Roost
Tailwind

What's the point of a Mild nature on Pidgeot? It specializes in all special attacks and needs all the Speed it can get. There's little point in investing in its bulk too. It also can't have Early Bird as an ability.

Anyway:

Regirock:

-Stealth Rock
-Rock Slide
-Drain Punch
-Thunder Wave
Nature: Careful
EVs: 252 HP/4 Def/252 SDef
Item: Leftovers

Tanks any of Mega Pidgeot's attacks and can dent or KO it with a guaranteed hit from Rock Slide.

 

[faf]

Spoiler:

Golurk @ Leftovers
Trait: Iron Fist
Evs: 248hp/252atk/8spd
Adamant nature (+Atk, -Sp. Atk)
-Stealth Rock
-Earthquake
-Shadow Punch
-Fire Punch

Immune to T-wave and tanks Regirock's offensive moves:
0 Atk Regirock Rock Slide vs. 248 HP / 0 Def Golurk: 48-57 (12.5 - 14.9%) -- possibly the worst move ever
(I was even factoring in the possible rocks Regi could set up)

252+ Atk Golurk Earthquake vs. 252 HP / 4 Def Regirock: 188-224 (51.6 - 61.5%) -- 97.7% chance to 2HKO after Leftovers recovery

EDIT: The Mega Pidgeot unfortunately doesn't even counter Mega Pinsir cause it can't switch into a Return:
252 Atk Aerilate Mega Pinsir Return vs. 0 HP / 0- Def Pidgeot: 331-391 (107.8 - 127.3%) -- guaranteed OHKO
Can it check it? Depends on the situation.
@PD- I think he meant 252atk/4spd/252spe.

 

[Nah]

You don't need to worry about entry hazards even if they're on the set Zinnia.

Mega Blastoise w/Blastoisite
Nature: Modest
Ability: Mega Launcher
EVs: 236 HP, 252 Sp.Attack, 20 Speed
Moves: Water Pulse, Dark Pulse, Aura Sphere, Ice Beam

252+ Atk Golurk Earthquake vs. 236 HP / 0 Def Mega Blastoise: 148-175 (41.3 - 48.8%) -- guaranteed 3HKO

252+ SpA Mega Launcher Mega Blastoise Water Pulse vs. 248 HP / 8 SpD Golurk: 396-468 (103.9 - 122.8%) -- guaranteed OHKO

 

[Detox]

Gastrodon @ Leftovers
Ability: Storm Drain
EVs: 252 HP / 248 SpD / 8 Spe
Calm Nature
- Scald
- Earth Power
- Toxic
- Recover

I'm too lazy to post calcs but nothing on the set can even 3HKO Gastrodon, while it simply Toxic stalls you.

 

[Master Kavar]

Tropius
Item pecha berry
Ability solar power
Evs sp.attk 252 sp.def 258 attk 8
Nature modest
Moves
Whirlwind
Leaf storm
Synthesis
Body slam

 

[Detox]

Bouffalant @ Choice Band
Ability: Sap Sipper
EVs: 172 HP / 252 Atk / 84 Spe
Adamant Nature
- Head Charge
- Earthquake
- Megahorn
- Stone Edge

AfroBull is immune to your primary offensive move, and doesn't really care too much about Body Slam.

8- Atk Tropius Body Slam vs. 172 HP / 0 Def Bouffalant: 43-51 (11.4 - 13.6%) -- possible 8HKO

252+ Atk Choice Band Bouffalant Stone Edge vs. 0 HP / 0 Def Tropius: 374-440 (110.3 - 129.7%) -- guaranteed OHKO

252+ Atk Choice Band Bouffalant Head Charge vs. 0 HP / 0 Def Tropius: 334-394 (98.5 - 116.2%) -- 93.8% chance to OHKO

BTW those EVs are impossible to have on a legitimate pokemon. The cap is 510 and you have 518

 

[Professor_Jared]

Sableye@Leftovers
Ability: Prankster
252 HP/252 Def/4 Special Def

Taunt
Will-O-Wisp
Foul Play
Recover

252+ Atk Choice Band Bouffalant Megahorn vs. 252 HP / 252+ Def Sableye: 165-195 (54.2 - 64.1%) -- guaranteed 2HKO after Leftovers recovery

252+ Atk Choice Band burned Bouffalant Megahorn vs. 252 HP / 252+ Def Sableye: 82-97 (26.9 - 31.9%) -- 39.7% chance to 4HKO after Leftovers recovery

Sableye prankster burns the Afro buffalo, heals off the potential Megahorn damage, and proceeds to stall or Foul Play it to death.

 

[rob_thijs]

Sylveon@choice specs
Ability: Pixilate
244 HP / 252 SpA / 12 Spe

Hyper Voice
Psyshock
Shadow Ball
HP Fire

0 Atk Sableye Foul Play vs. 244 HP / 0 Def Sylveon: 46-54 (11.7 - 13.7%) -- possible 8HKO

252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 252 HP / 4 SpD Sableye: 788-930 (259.2 - 305.9%) -- guaranteed OHKO

You can burn if you want but you can't stall.

 

[Megan]

Bastiodon @ Leftovers
EVs: 248 HP / 8 Def / 252 SpD
Ability: Sturdy
Careful Nature
- Heavy Slam
- Toxic
- Earthquake
- Magic Coat

252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 248 HP / 252+ SpD Bastiodon: 80-95 (24.7 - 29.4%) -- possible 5HKO after Leftovers recovery


0 Atk Bastiodon Heavy Slam (120 BP) vs. 244 HP / 0 Def Sylveon: 218-260 (55.6 - 66.3%) -- guaranteed 2HKO

 

[Nah]

Sandslash w/Lum Berry
Nature: Adamant
Ability: Sand Rush
EVs: 172 HP, 252 Attack, 84 Speed
Moves: Earthquake, Knock Off, Rapid Spin, Rock Tomb

0 Atk Bastiodon Heavy Slam (120 BP) vs. 172 HP / 0 Def Sandslash: 72-85 (21.5 - 25.4%) -- 0.3% chance to 4HKO

 

[Detox]

Swampert @ Leftovers
Ability: Damp
EVs: 240 HP / 16 Atk / 252 Def
Relaxed Nature
- Stealth Rock
- Earthquake
- Scald
- Protect

252+ Atk Sandslash Earthquake vs. 240 HP / 252+ Def Swampert: 117-138 (29.1 - 34.4%) -- 99.7% chance to 4HKO after Leftovers recovery

0 SpA Swampert Scald vs. 172 HP / 0 SpD Sandslash: 242-288 (72.4 - 86.2%) -- guaranteed 2HKO

 

[Omicron]

Tangrowth @ Assault Vest
Ability: Regenerator
EVs: 252 HP / 4 Def / 252 SpD
Sassy Nature
- Giga Drain
- Knock Off
- Earthquake
- Leaf Storm

0 SpA Tangrowth Leaf Storm vs. 240 HP / 0 SpD Swampert: 664-784 (165.5 - 195.5%) -- guaranteed OHKO

0 SpA Swampert Scald vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 27-33 (6.6 - 8.1%) -- possibly the worst move ever
16 Atk Swampert Earthquake vs. 252 HP / 4 Def Tangrowth: 49-58 (12.1 - 14.3%) -- possible 7HKO

 

[rob_thijs]

I did not see anything about not using mega's so here it is.

Charizard @ Charizardite Y
Ability: Blaze
EV's: 4 Def / 252 SpA / 252 Spe
Timid
- Fire Blast
- Solar Beam
- Focus Blast
- Air Slash

0 Atk Tangrowth Knock Off vs. 0 HP / 4 Def Mega Charizard Y: 57-68 (19.1 - 22.8%) -- possible 5HKO
252 SpA Mega Charizard Y Fire Blast vs. 252 HP / 252+ SpD Assault Vest Tangrowth in Sun: 452-534 (111.8 - 132.1%) -- guaranteed OHKO

If mega's are not allowed regular Charizard will still be able to take him out in two hits
0 Atk Tangrowth Knock Off vs. 0 HP / 4 Def Charizard: 57-68 (19.1 - 22.8%) -- possible 5HKO
252 SpA Choice Specs Charizard Fire Blast vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 344-408 (85.1 - 100.9%) -- 6.3% chance to OHKO

 

[Nah]

I did not see anything about not using mega's so here it is.

Anything is allowed so long as it's not in the Uber tier.

As for the Zard Y counter:

Garchomp w/Lum Berry
Nature: Jolly
Ability: Rough Skin
EVs: 252 Attack, 252 Speed, 4 HP
Moves: Dragon Claw, Earthquake, Stone Edge, Swords Dance

Can survive all of Mega Zard Y's attacks, then outspeeds and OHKOs with Stone Edge.

 

[Omicron]

Blastoise @ Blastoisinite
Ability: Rain Dish
EVs: 56 HP / 252 SpA / 200 Spe
Modest Nature
- Hydro Pump
- Dark Pulse
- Aura Sphere
- Dragon Pulse

252+ SpA Mega Blastoise Hydro Pump vs. 0 HP / 0 SpD Landorus-T: 488-576 (152.9 - 180.5%) -- guaranteed OHKO

252 Atk Landorus-T Earthquake vs. 236 HP / 0 Def Mega Blastoise: 153-180 (42.7 - 50.2%) -- 0.4% chance to 2HKO

 

[rob_thijs]

I like this one:)

Lanturn @ Choice Specs
Ability: Volt Absorb
196 HP / 252 SpA / 60 SpD
- Scald
- Thunderbolt
- Ice Beam
- HP Grass

252+ SpA Mega Blastoise Hydro Pump vs. 196 HP / 60 SpD Lanturn: 118-139 (26.8 - 31.5%) -- guaranteed 4HKO
252+ SpA Mega Launcher Mega Blastoise Dragon Pulse vs. 196 HP / 60 SpD Lanturn: 181-214 (41.1 - 48.6%) -- guaranteed 3HKO
252+ SpA Mega Launcher Mega Blastoise Dark Pulse vs. 196 HP / 60 SpD Lanturn: 172-203 (39 - 46.1%) -- guaranteed 3HKO
252+ SpA Mega Launcher Mega Blastoise Aura Sphere vs. 196 HP / 60 SpD Lanturn: 172-203 (39 - 46.1%) -- guaranteed 3HKO

252+ SpA Choice Specs Lanturn Thunderbolt vs. 56 HP / 0 SpD Mega Blastoise: 302-356 (96.4 - 113.7%) -- 75% chance to OHKO

This is the set for my Lanturn I use in my UU team. Lanturn will probably always move last to the pokémon he face, that's why I invested 60 in SpD instead of Spe and it works very well.
For example he lives this:
252 SpA Life Orb Roserade Leaf Storm vs. 196 HP / 60 SpD Lanturn: 166-198 (37.7 - 45%) -- guaranteed 3HKO

 

[Master Kavar]

Torterra @ leftovers
Ability: Shell armor
Nature sassy
EV's 252 Sp.def / 251 / sp. attk / 7 attk
- Grass Pledge
- Synthesis
- Outrage
- Grassy Terrain