You won't be able to get this until later in the game. An elite guards is very different from the shield guys you encounter early in the game. Elite guards have to be stunned before they're able to be killed.
The very first one you come across will be a simple kill. Without giving any spoilers, you have to sneak around him without being noticed. On the other side of him is an electrical current; all you have to do is press the button and the guard will come and get shocked this will stun him. Then proceed with the Kill!