No Guard ensures that all attacks used by, and targeted at, the ability-bearer hit without fail.
Headbutt is Normal type, but it hit your Honedge because it had the ability No Guard.
As your comment below you have said that Scratch and Tackle do not have any effect on the Honedge, so, either I am misinterpreting how No Guard works, or the Zigzagoon used Scratch and Tackle and then used Odor Sleuth.
>Resets opponent's Evasiveness, Normal-type and Fighting-type attacks can now hit Ghosts, and Ghost-type attacks hit Normal.
It must have used Scratch and Tackle then used Odor Sleuth.
Hope I helped!