I'm sure there's a one Pokemon core out there, but I'm not going to find it by tomorrow. Miltank is free if anyone wants to use it on the way. Chople was just not good enough to work.
Shellos@Eject Button
Ability - Sticky Hold
Nature - Bold (+Def, -Atk)
252 HP / 252 Def
~Toxic
~Yawn
~Recover
~Whirlpool
Torterra@Choice Scarf
Ability - Shell Armor
Nature - Jolly (+Spe, -SpA)
252 Atk / 76 Def / 180 Spd
~Earthquake
~Quake of Earth
~More Quake Plz
~Frenzy Plant
Very simple core. Shellos comes in, lives a hit (and is thus not death fodder), and gets switched out by Eject Button. Torterra then comes in and kills everything with Earthquake, taking a Sucker Punch from Skuntank if necessary. Shellos cannot be tricked because of Sticky Hold and can annoy the core if they try to simply not attack it and switch back and forth to prevent Torterra getting a free switch in (spam Toxic if Skuntank is in, spam Whirlpool if Jynx is in, or Toxic if it locks itself into Trick; or really just spam Toxic). 180 Speed EVs give Torterra 212 Speed, letting it outspeed Jynx by 1 after Choice Scarf.
252Atk Skuntank (+Atk) Poison Jab vs 252HP/252Def Shellos (+Def): 35% - 42% (127 - 150 HP). Guaranteed 3HKO.
252SpAtk Choice Specs Jynx (Neutral) Psyshock vs 252HP/252Def Shellos (Neutral): 60% - 71% (217 - 256 HP). Guaranteed 2HKO. (accidently done with neutral Defense nature, but that doesn't matter)
252SpAtk Choice Specs Jynx (Neutral) Focus Blast vs 252HP/0SpDef Shellos (Neutral): 74% - 87% (265 - 312 HP). Guaranteed 2HKO. (Shellos is pure Water so Ice Beam does less than this)
252Atk Torterra (Neutral) Earthquake vs 20HP/0Def Skuntank (Neutral): 114% - 134% (402 - 474 HP). Guaranteed OHKO.
252Atk Torterra (Neutral) Earthquake vs 0HP/0Def Dry Skin Jynx (Neutral): 118% - 139% (322 - 379 HP). Guaranteed OHKO.
252Atk Skuntank (+Atk) Sucker Punch vs 0HP/76Def Torterra (Neutral): 30% - 36% (102 - 121 HP). Guaranteed 4HKO.
Shellos@Eject Button
Ability - Sticky Hold
Nature - Bold (+Def, -Atk)
252 HP / 252 Def
~Toxic
~Yawn
~Recover
~Whirlpool
Torterra@Choice Scarf
Ability - Shell Armor
Nature - Jolly (+Spe, -SpA)
252 Atk / 76 Def / 180 Spd
~Earthquake
~Quake of Earth
~More Quake Plz
~Frenzy Plant
Very simple core. Shellos comes in, lives a hit (and is thus not death fodder), and gets switched out by Eject Button. Torterra then comes in and kills everything with Earthquake, taking a Sucker Punch from Skuntank if necessary. Shellos cannot be tricked because of Sticky Hold and can annoy the core if they try to simply not attack it and switch back and forth to prevent Torterra getting a free switch in (spam Toxic if Skuntank is in, spam Whirlpool if Jynx is in, or Toxic if it locks itself into Trick; or really just spam Toxic). 180 Speed EVs give Torterra 212 Speed, letting it outspeed Jynx by 1 after Choice Scarf.
252Atk Skuntank (+Atk) Poison Jab vs 252HP/252Def Shellos (+Def): 35% - 42% (127 - 150 HP). Guaranteed 3HKO.
252SpAtk Choice Specs Jynx (Neutral) Psyshock vs 252HP/252Def Shellos (Neutral): 60% - 71% (217 - 256 HP). Guaranteed 2HKO. (accidently done with neutral Defense nature, but that doesn't matter)
252SpAtk Choice Specs Jynx (Neutral) Focus Blast vs 252HP/0SpDef Shellos (Neutral): 74% - 87% (265 - 312 HP). Guaranteed 2HKO. (Shellos is pure Water so Ice Beam does less than this)
252Atk Torterra (Neutral) Earthquake vs 20HP/0Def Skuntank (Neutral): 114% - 134% (402 - 474 HP). Guaranteed OHKO.
252Atk Torterra (Neutral) Earthquake vs 0HP/0Def Dry Skin Jynx (Neutral): 118% - 139% (322 - 379 HP). Guaranteed OHKO.
252Atk Skuntank (+Atk) Sucker Punch vs 0HP/76Def Torterra (Neutral): 30% - 36% (102 - 121 HP). Guaranteed 4HKO.