^nitpicking but Nidoking always wins. You forgot to factor in Sheer Force when doing the calculation.
252Atk Rock Gem +2
Terrakion (Neutral)
Stone Edge vs 4HP/0Def
Nidoking (Neutral): 99% - 117% (302 - 356 HP). Guaranteed 2HKO. 96% chance to OHKO.
252SpAtk Life Orb Sheer Force
Nidoking (Neutral)
Earth Power vs 4HP/0SpDef
Terrakion (Neutral): 125% - 148% (408 - 482 HP). Guaranteed OHKO.
Scarf Nidoking can outspeed and OHKO
252SpAtk Sheer Force
Nidoking (Neutral)
Earth Power vs 4HP/0SpDef
Terrakion (Neutral): 96% - 114% (314 - 372 HP). Guaranteed 2HKO. 82% chance to OHKO.
EDIT: Gliscor does not beat SubSD Terrakion. Here is why:
252Atk Rock Gem +2
Terrakion (Neutral)
Stone Edge vs 252HP/184Def Leftovers
Gliscor (+Def): 88% - 105% (315 - 372 HP). Guaranteed 2HKO. 31% chance to OHKO.
0Atk
Gliscor (Neutral)
Earthquake vs 4HP/0Def Leftovers
Terrakion (Neutral): 69% - 82% (224 - 266 HP). Guaranteed 2HKO.
So only Golurk, Nidoqueen and Scarf Nidoking beat Terrakion 1vs1.
Edit 2: @PenguinX, the onsite Golurk is eved to avoid the 2hko from CB Earthquake and KO back with Earthquake.
EDIt 2: @ PenguinX again....Golurk is the only counter for real. It actually works. Look at edit 2. It beats every Terrakion set and this isn't a joke. Go check out the analysis maybe? And please only assume SR please. We would actually have to rewrite alot of the checks and counters section if we assumed SR and spikes.
252Atk Choice Band
Terrakion (Neutral)
Earthquake vs 252HP/176Def Leftovers
Golurk (+Def): 38% - 45% (146 - 172 HP). Guaranteed 3HKO.
252Atk Rock Gem +2
Terrakion (Neutral)
Stone Edge vs 252HP/176Def Klutz
Golurk (+Def): 56% - 67% (217 - 256 HP). Guaranteed 2HKO.
Leftovers prevents the KO from choice band while in return,
80Atk Iron Fist
Golurk (Neutral)
Earthquake vs 4HP/0Def Leftovers
Terrakion (Neutral): 94% - 111% (306 - 360 HP). Guaranteed 2HKO. 66% chance to OHKO. Guaranteed KO with SR.
http://www.smogon.com/bw/pokemon/golurk/ou
Note that Golurk isnt KOed by +2 Rock Gem Stone Edge even with 3 layers of spikes and SR.