This can be done solo as I unlocked it that way, but it obviously requires some pretty heavy RNG. I only got to knife 2 or 3 enemies in my whole playthrough of the campaign that way.
So co-op is your best bet. Load up a chapter on Easy and have a friend perform a Yank move, by taking cover opposite of an enemy in cover, pushing up on the
and then
when the prompt appears in the bottom middle of the screen. Then, as the enemy is stunned, get up close and take the kill by pressing
, again when the prompt appears.
EDIT: As NAPPSTER187 reminded me, this can also be unlocked in a private match with two of your friends.