Here I Come to Save the Day, can be unlocked early in the Game.
After Rebecca and Billy Team up in the Dining Car, make your way to the Passenger Car.
Move to the middle of the Room and let the Zombies behind and in front get up. Jump into your Partner Menu and select IDLE.
Shot the Zombie in front of you and by now the other Zombie should attack and grab your Partner.
Start Shooting him and as soon as he lets go the Achievement unlocks.