This achievement is called Paper, Rock, Scissors. To win the achievement, you must press Y near your partner and not in combat to have the mini-game function. You must then win the mini-game 3 times in a row against your partner for the achievement to unlock. I am unsure if you must have a human partner or this can be done with the AI. I did this with a human partner though. My partner and I just had to choose the same option three times in a row and then do the same for my partner to win unlock it.