Cor travels and fights with you en route to each stages boss fight. To earn this achievement you must perform 5 blindside link attacks. You accomplish these by running behind the enemy and pressing
, If Cor is within striking distance then he will link up performing this special move.
I found it easier on larger enemies, but as long as you keep running behind the enemy and attacking you will quickly get this.
NOTE. It must say Blindside Link, if it does not say link then you have just performed a solo blindside and that does not count