Who Do You Trust? - 30
Build up a certain level of trust with your partner.
Here's how I got the achievement. On the level where you are walking on a conveyor belt, there are zombies that are lying down. If you get too close to them they lunge at you and you need to shake them off or have Sheva free you. Let yourself get grabbed and then thank Sheva everytime she saves you. I did this about 6-7 times and the achievement unlocked.