@CrashinBoomBang: Kadabra can't switch in on any attack from Torterra if it has set up once with Rock Polish, as it will be outsped and KO'd.
Serperior @ Leftovers
EVs: 144 HP / 144 Def / 220 SDef
- Calm Mind
- Hidden Power [Ice]
- Giga Drain
This Serperior has exactly enough physical bulk to survive two Stone Edges from a +2 speed Torterra on the switch-in, and enough special bulk to survive anything but a max roll Draco Meteor (so 93,75% chance to survive) when switching in on Altaria. It has enough natural speed uninvested to outspeed Modest Altaria.
- If it switches in on Torterra's Stone Edge, it will proceed to use Synthesis to regain health while Torterra wears itself down with Life Orb; after three uses of Synthesis, Serperior will be able to KO with HP Ice, and survive with enough HP to take Altaria's attack when it comes in after using another Synthesis (as it outspeeds Modest Altaria)
- If it switches in on Altaria's Draco Meteor and lives (which it will 15 out of 16 times), it will use Synthesis once, then it can Calm Mind twice without being KO'd by further Meteors, and proceed to 1hko Altaria and the unboosted Torterra (which it also outspeeds) with HP Ice.
- If it switches in on Altaria's Dragon Pulse, it alternates between Synthesis and Calm Mind twice, then the result is the same as if it switched in on Draco Meteor.
All those scenarios assume Torterra/Altaria don't switch, but the counter is (probably) still valid if they do switch between each other as it gives Serperior free turns to recover or set up. They also work assuming max damage from any attack EXCEPT switching in on Altaria's Draco Meteor, and also assuming min rolls on HP Ice's damage.
Thus this set (though its viability is perhaps questionable) should be able to counter the core 15/16 of the time, as the only thing it can't switch into is a max roll of Altaria's Draco Meteor. It is, however, vulnerable to crits, as it relies on taking and recovering from a few attacks before defeating the core. On the other hand, it also gets a little leeway between non-max rolls and the possibility of Stone Edge or Draco Meteor missing.
252 Atk Life Orb Torterra (+Atk) Stone Edge vs 144 HP/144 Def Serperior (+Def) : 34,56% - 40,67% (3-4 hits to KO)
252 SpAtk Choice Specs Altaria (+SpAtk) Draco Meteor vs 144 HP/220 SpDef Serperior: 64,22% - 75,54% (2 hits to KO)
252 -2 SpAtk Choice Specs Altaria (+SpAtk) Draco Meteor vs 144 HP/220 SpDef Serperior: 32,11% - 37,92% (3-4 hits to KO)
252 SpAtk Choice Specs Altaria (+SpAtk) Dragon Pulse vs 144 HP/220 SpDef Serperior: 41,28% - 48,93% (3 hits to KO)
0 SpAtk Serperior Hidden Power Ice vs 4 HP/0 SpDef Torterra: 55,42% - 66,27% (2 hits to KO)
0 SpAtk Serperior Hidden Power Ice vs 0 HP/0 SpDef Altaria: 53,61% - 63,23% (2 hits to KO)
I hope this is a good enough format and explanation as to why and how this Pokémon counters the core.