Well, it's not a story related achievement but you'll get it as a part of training in the beginning of the game. As soon as you kill your very first opponent - the achievement is yours.
But one word of warning and that's the actual purpose of posting this solution on such easy achievement. For me the cheevo denied to pop after that first kill. I reloaded the game and it didn't work again. But as soon as I reloaded steam the achievement popped immediately. So you know what to do if it happen to you!