One thing to notice: It's okay to back down. If you feel like you're spotted and the enemy has the perfect line of sight on you, drop down or hide for a few seconds. Normally the enemy will stay at the same position (not in higher ranks, and here some might start to push onto you) and then you can jump in, surprising the enemy to get the kill. Other than that, you just have to keep on practicing. I don't know how different it is on console but just constantly practice your aim with the controller to get the feel on always hitting the head or at least the body (hitting body shots is also okay too)