This Trophy is for saving your partner when they're being attacked by an enemy. This can be earned as early as the train. As soon as you have Billy and Rebecca together, go through the automatic door near the back of the train. Pass through the kitchen and you'll be in a storage area with some dog kennels. You can try to let one of the dogs grab your partner here, but I'd suggest returning through the kitchen. A zombie will burst out of the fridge on your way back. Let it grab your partner, then just take a few shots at it with your handgun to unlock the Trophy.