Does 0 damage count as damage? One-Two saying you can use it to clear 6 Weakness tokens suggests yes, but Knockdown Style suggests that reducing damage to 0 with Armor and Iron means it doesn't count as damage.
0 damage does not count as "damage," no. One-Two is going to have a written in exception in the errata I am working on, something like: "When you deal damage to an enemy with an Action, hit them again for 1 more damage. Even if the initial damage is reduced to zero by Armor, Iron tokens, or Weakness tokens, you still get this extra 1-damage hit."