Scyther is faster than all of those. With one Swords Dance and Technician:
Max HP/Def +Def Gastly taking Aerial Ace: 136.99% - 160.27%
Max HP/Def +Def Drifloon taking Aerial Ace: 93.20% - 109.71%
Max HP/Def +Def Koffing taking Aerial Ace: 74.36% - 87.18%
Max HP/Def +Def Geodude taking X-Scissor: 64.10% - 75.64%
Max HP/Def +Def Onix taking X-Scissor: 48.68% - 57.89%
Max HP/Def +Def Starly taking Aerial Ace: 128.21% - 150.00%
Koffing, Geodude, Shieldon, Drifloon, Aron, Onix survive. Of all of those, only Koffing, Shieldon, Aron, and possibly Onix can get past Focus Sash. Koffing with WoW, Onix by the 3HKO chance. Geodude will activate Sash and faint on the next turn. Onix, Shieldon and Aron can Roar. Onix and Koffing will take a huge chunk of health from this, and Onix has only a possibility of living to Stone Edge again, so it will most likely Roar, meaning it got rid of a full-health Scyther by taking around half health. You can Aromatherapy off the burn and Koffing dies on the next turn anyways. It has no way of OHKOing Scyther, even if it doesn't have Sash. By the very definition of a counter, it must be able to switch into a threat and threaten it or faint it with little or no risk to itself. This leaves, again, Aron and Shieldon.