This achievement can be earned a few ways. The way I found easiest was using the crossbow to headshot enemies not looking. Try not to shoot anyway if there is others around because they will see the dead body. The other way is by sneaking up behind an unaware enemy and hit ( Y ) the should put them down. After 25 unaware kills your 15 Gamerscore will be yours. Good Luck. Message for any help if needed.