Call of Duty Advanced Warfare Carma Achievement/Trophy will give you one Bronze Trophy or 10 GamerScore points depending upon your console. You need to finish your enemy by throwing a car door at them.
To get the Call of Duty Advanced Warfare: Carma Achievement/Trophy go to the street with a high-tech tank on the left side in Mission 1: Induction, you will get to this street quite early in the mission.
On this street you will find some vehicles with open doors. To use these doors as your lethal weapon, pull them off those vehicles. You need to get hold of the door before moving on to the next area. When you reach a building full of enemies, just throw the door that you are holding, at one of the enemy.
If you are still unsure, check out this achievement video given below: