Chapter 17: For Better or Worse (Encounter: Get to the Car)
During the first enemy encounter in chapter 17 you must steal a car. Neither you, nor Elena can kill anyone! If you are detected Elena will kill enemies. So you must do it in stealth. Climb around the cliffside and use the rope to get past the enemies unnoticed. On Crushing difficulty they will likely spot you, so put the difficulty to explorer.
Drive off in the car and make sure you don't run over anyone. If Elena shoots someone during the car ride it will count as a kill too. So do your best to enter the car unnoticed and it should be fine.