Counter:
Carracosta @ Leftovers
Ability: Solid Rock
EVs: 252 HP / 192 Def / 64 SpD
Relaxed Nature
- Stealth Rock
- Scald
- Toxic
- Stone Edge / Protect / Aqua Jet
EQ is a 4HKO against Defensive Carracosta, whereas Carracosta can Stone Edge for the kill or an easy 2HKO (or OHKO with rocks) with Scald. If you are content with the Scald then the last option can be a Protect to ensure the 4HKO after rocks, or Aqua Jet for another 2HKO that can bypass Archeop's Speed and grab it before the Roost.
0 SpA Carracosta Scald vs. 0 HP / 0 SpD Archeops: 210-248 (72.1 - 85.2%) -- 68.8% chance to OHKO after Stealth Rock
252 Atk Archeops Earthquake vs. 252 HP / 192+ Def Solid Rock Carracosta: 106-126 (30.1 - 35.7%) -- guaranteed 4HKO after Leftovers recovery
Additionally the Shell Smash variant can also check Archeops. Aqua Jet is a 2HKO even without setup, and Stone Edge is definitely a KO, but Archeops actually does enough damage with EQ that Carracosta can use it to set up potentially...unless there's a Taunt.
252+ Atk Carracosta Aqua Jet vs. 0 HP / 0 Def Archeops: 182-216 (62.5 - 74.2%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Carracosta Stone Edge vs. 0 HP / 0 Def Archeops: 450-530 (154.6 - 182.1%) -- guaranteed OHKO
252 Atk Archeops Earthquake vs. 0 HP / 0 Def Solid Rock Carracosta: 135-160 (46.7 - 55.3%) -- 73.4% chance to 2HKO
So yeah, this mon is a counter and a check I suppose.