Counter that Pokémon

Rotom-W @ Choice Scarf
Trait: (No Ability)
EVs: 4 HP / 252 SAtk / 252 Spd
Timid Nature (+Spd, -Atk)
- Volt Switch
- Hydro Pump
- Hidden Power [Ice]
- Trick

Trick cripples Hippowdon.

252 SpA Rotom-W Hydro Pump vs. 252 HP / 252+ SpD Hippowdon: 272-324 (64.7 - 77.1%)
252 SpA Rotom-W Hidden Power Ice vs. 252 HP / 252+ SpD Hippowdon: 102-120 (24.2 - 28.5%)

Shaymin @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 80 SpA / 176 SDef
Modest Nature (+SpA, -Atk)
- Earth Power
- Rest
- Seed Flare
- Hidden Power [Fire]

Owns Rotom-W with Seed Flare and doesn't really mind any of the attacks or being Tricked

Dragonite @ Leftovers
Trait: Multiscale
EVs: 252 Atk / 252 Spe / 4 HP
Adamant Nature
- Dragon Dance
- Dragon Claw
- Fire Punch
- Iron Head

Dragonite either resists or is immune to all of Shaymin's attacks while either powering up or dealing super effective damage back.

Porygon2 @ Eviolite
Trace (Multiscale)
252 HP / 4 Def / 252 SAtk
Modest
-Thunderbolt
-Ice Beam
-Thunder Wave
-Recover

Offensive calcs
252+ SpA Porygon2 Ice Beam vs. 4 HP / 0 SpD Multiscale Dragonite: 186-220 (57.4 - 67.9%)
252+ SpA Porygon2 Ice Beam vs. 4 HP / 0 SpD Dragonite: 372-440 (114.8 - 135.8%)

Defensive calcs
252+ Atk Dragonite Dragon Claw vs. 252 HP / 4 Def Eviolite Multiscale Porygon2: 54-63 (14.4 - 16.8%)
252+ Atk Dragonite Dragon Claw vs. 252 HP / 4 Def Eviolite Porygon2: 108-127 (28.8 - 33.9%)

Machamp w/ Lum Berry
Ability: No Guard
EVs: 240 HP / 248 Atk / 16 SpD / 4 Spe
Nature: Adamant
~ DynamicPunch
~ Payback
~ Bullet Punch
~ Stone Edge

Machamp's got good Attack stat, and thanks to No Guard, it should be able to land all its moves without worrying about anything. With only about two to three DynamicPunches, Porygon2 would instantly be taken down.

Slowbro @ Lefties
Regenerator
248 HP / 244 Def / 16 SpA
Bold nature
-Scald
-Psyshock
-Thunder Wave
-Slack Off

Best options against Slowbro are Payback and Stone Edge, which do about 25.9 - 30.5% only. Scald has a nice burn rate to cripple it and Psyshock 2HKO's Machamp.

Hydreigon (M) @ Life Orb
Trait: Levitate
EVs: 4 HP / 252 SAtk / 252 Spd
Modest Nature (+SAtk, -Atk)
- Draco Meteor
- Focus Blast
- Fire Blast
- Dark Pulse

Sylveon @ Lefties
Cute Charm
252 HP / 16 SpA / 248 SpD
Careful nature
-Calm Mind
-Moonblast
-Shadow Ball
-Wish

I never really used Sylveon, but I'll just go with this set. Takes little to no damage from Hydreigon, Moonblast OHKOs.

Scizor (F) @ Iron Plate
Trait: Technician
EVs: 252 HP / 96 Atk / 160 SDef
Adamant Nature (+Atk, -SAtk)
- Swords Dance
- U-turn
- Bullet Punch
- Roost

Kills Sylveon handily with BP and doesn't take much from its atks, boosted or not.

Heatran @ Leftovers
Flash Fire
4 HP / 252 SAtk / 252 Spd
Timid nature
-Fire Blast
-Earth Power
-Hidden Power Ice
-Taunt

Common Heatran set, outspeeds Scizor and proceeds to KO with Fire Blast.

Gyarados @ Lefties
Moxie
88 HP / 252 Atk / 168 Spe
Adamant
-EQ
-Waterfall
-Dragon Dance
-Bounce

Can withstand all of Heatran's attacks and OHKO's with EQ.

Rotom-W @ Lefties
Levitate
248 HP / 252 Def / 8 SAtk
Bold nature
-Hydro Pump
-Volt Switch
-Will-O-Wisp
-Pain Split

self-explanatory :]

Shaymin @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 4 Def / 252 SAtk
Modest Nature (+SAtk, -Atk)
- Seed Flare
- Substitute
- Hidden Power [Fire]
- Protect

-Talonflame w/Life Orb
Nature: Adamant
Ability: Gale Wings
EVs: 252 Attack, 126 Speed, 126 HP
Moves: Flare Blitz, Brave Bird, U-turn, Roost?

Seed Flare and HP Fire will do little to it. Talonflame then easily OHKOs with Flare Blitz or Brave Bird. Substitute and Protect won't save you.

Barbaracle:

-Shell Smash
-Stone Edge
-Razor Shell/Earthquake
-Cross Chop/Poison Jab
Nature: Jolly/Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: White Herb/Lum Berry
Ability: Tough Claws

Barbaracle resists both of Talonflame's STABs, while taking a pittance from U-turn. It then sets up with Shell Smash and sweeps.

Venusaur:
Ability: Chlorophyll (Thick Fat)
IVs: 252 HP / 4 Def / 252 SpDef
Nature: Bold
- Synthesis
- Giga Drain
- Leech Seed
- Sleep Powder

Can take even a +2 Stone Edge and obliterate with Giga Drain along with regaining HP.

Heatran w/ Choice Scarf
Ability: Flash Fire
Nature: Modest
112 HP / 160 SpA / 236 Spe
~ Overheat
~ Earth Power
~ Dragon Pulse
~ Flamethrower

Heatran is able to endure all Giga Drains that are thrown at it. Thanks to Scarf, it is guaranteed to out-speed Venusaur. And with only a couple of Overheat or Flamethrower attacks, Heatran could take out Venusaur without any sort of issues.

Believe it or not, Mega Venusaur kinda walls Heatran thanks to Thick Fat.

160+ SpA Heatran Flamethrower vs. 252 HP / 252 SpD Thick Fat Mega Venusaur: 108-128 (29.6 - 35.1%)
160+ SpA Heatran Overheat vs. 252 HP / 252 SpD Thick Fat Mega Venusaur: 152-182 (41.7 - 50%)

It can probably stall out Heatran with Sleep Powder, Leech Seed, and Synthesis.


160+ SpA Heatran Earth Power vs. 212 HP / 0 SpD Azumarill: 122-144 (30.9 - 36.5%)
252+ Atk Choice Band Huge Power Azumarill Waterfall vs. 112 HP / 0 Def Heatran: 456-536 (129.9 - 152.7%)
252+ Atk Choice Band Huge Power Azumarill Aqua Jet vs. 112 HP / 0 Def Heatran: 228-270 (64.9 - 76.9%)

The guy didn't say if Venusaur was holding onto a Venusaurite or not, so I was kind of confused. xD" Anyway…

Celebi w/ Leftovers
Ability: Natural Cure
Nature: Calm
252 HP / 236 SpD / 20 Spe
~ Giga Drain
~ Recover
~ U-turn
~ Thunder Wave

Celebi could slow down Azumarill with Thunder Wave, then steal all its health with Giga Drain. If it's ever in low HP, it could always restore itself with Recover. When it's in danger, it could instantly switch out with U-turn.

-Mega Charizard Y w/Charizardite Y
Nature: Modest
Ability: Drought
EVs: 252 Sp.Attack, 252 Speed, 4 Defense
Moves: Fire Blast/Flamethrower, Focus Blast, Solar Beam, Air Slash/Dragon Pulse

Megazard Y is practically uneffected by Giga Drain and U-turn, and Thunder-Wave doesn't help Celebi against it. Then it can KO Celebi with a sun-boosted Fire attack.