My previous idea was Rotom-F, which only worked about 50% of the time:
...

Rotom-F
Rotom-F @ Choice Scarf
Timid Nature
EV's: 8 HP / 252 SpAtk / 248 Spe
-Blizzard
-Thunderbolt
-Volt Switch
-Trick
Rotom-F is a 'mon that I already regularly run that I would be comfortable sending out against this core, so that's who I'm going with. Sadly, he's outright OHKO'd by Torterra's Stone Edge and Wood Hammer, as well as Altaria's Draco Meteor. The good thing is that he can come in on Torterra's Rock Polish or Earthquake, as well as Altaria's Dragon Pulse, HP Ground, or Rest and proceed to outspeed and OHKO both of them with Blizzard. A shaky counter, but a counter nonetheless.
Calcs:
Torterra:
252 Atk Life Orb Torterra (+Atk) Wood Hammer vs 8 HP/0 Def Rotom-F: 96.3% - 113.58%
Entry hazards damage: 60
After entry hazards: 294 - 336 (120.99% - 138.27%)
Guaranteed OHKO
Switching into Wood Hammer is a bad idea...
252 Atk Life Orb Torterra (+Atk) Stone Edge vs 8 HP/0 Def Rotom-F: 107% - 125.93%
Entry hazards damage: 60
After entry hazards: 320 - 366 (131.69% - 150.62%)
Guaranteed OHKO
As is Stone Edge.
However, getting in on Rock Polish or Earthquake means you can take Torterra down easily.
252 SpAtk Rotom-F Blizzard vs 4 HP/0 SpDef Torterra: 234.94% - 276.51%
Guaranteed OHKO
Altaria:
252 SpAtk Choice Specs Altaria (+SpAtk) Draco Meteor vs 8 HP/0 SpDef Rotom-F: 97.53% - 114.81%
Entry hazards damage: 60
After entry hazards: 297 - 339 (122.22% - 139.51%)
Guaranteed OHKO
So you can't come in on Draco Meteor, but...
252 SpAtk Choice Specs Altaria (+SpAtk) Dragon Pulse vs 8 HP/0 SpDef Rotom-F: 62.96% - 74.07%
Entry hazards damage: 60
After entry hazards: 213 - 240 (87.65% - 98.77%)
2 hits to KO
You have just enough bulk to switch into Dragon Pulse.
252 SpAtk Rotom-F Blizzard vs 0 HP/0 SpDef Altaria: 222.68% - 263.92%
Guaranteed OHKO
So Rotom-F has a decent shot at beating the core, although it's only going to work about 50% of the time (probably less as I don't see Altaria using Rest or Dragon Pulse very often). I think Kadabra has been the best idea, although FLCL's Rain core looked like it would work as well. Anyone that wanted a standard 'mon could easily just pick up Fridge-tom though.
I'd like to add two separate entries at this point: one that's a little gimmicky, and one that's competitively viable.
First, the one that I think is a little gimmicky, which would be Abra. Cbb suggested it to me on IRC, and it looks like it works exactly the same as Kadabra does. With a Modest nature, Abra hits just as hard as Timid Kadabra does, and by extension can still OHKO both members of the core. Magic Guard prevents entry hazard damage and Focus Sash ensures that it will survive at least one hit (in the case of Torterra using Rock Polish to speed up and hit Abra).
Abra
Abra @ Focus Sash
Ability: Magic Guard
Modest Nature
EV's: 4 HP / 252 SpAtk / 252 Spe
-Hidden Power Ice
-Psychic
-Shadow Ball
-Disable (Why am I listing moves other than HP Ice?)
Calcs:
252 SpAtk Abra (+SpAtk) Hidden Power Ice vs 4 HP/0 SpDef Torterra: 100% - 118.07%
Guaranteed OHKO
252 SpAtk Abra (+SpAtk) Hidden Power Ice vs 0 HP/0 SpDef Altaria: 96.22% - 114.09%
81.25% chance to OHKO
Abra can easily switch into any attack that either 'mon can fire off, after which it can outspeed both and OHKO both. Altaria will survive 18.75% of the time, but that's about as often as Draco Meteor and Stone Edge will miss, so I don't see a problem with that. I don't really want to take full credit for this, as it was suggested to me by someone else and also kind of rips off Kadabra, so I'd also like to give an original suggestion, which I will edit in shortly.