Nix is the little Buddy/Companion you have with you throughout the game. You need to press
+
which should give you an option to "attack" on an enemy.
Nix should then jump on them and temporarily blind them by covering their face. You then NEED to let the animation finish for this to count as if you attack the enemy during it, it wont count.
You can do this to the same enemy over and over again, but it is important to let the animation to finish for it to count.