Sleeping Espinas Question

So after progressing a bit I found Green Nargacuga which gave me Assassins tail for a 1 hit KO move. I had ALSO fought a random espinas in a rare den and had that move trigger allowing an easy win. However when I tried this with the sleeping Espinas in Azuria for quite some time, it doesn’t seem to want to go down.

Am I horribly unlucky or is this specific Espinas immune to one hit k-o moves?