I actually got this inadvertently when trying to get the 3 for 1 with a RC bomb car. During the shrouded bunker mission, after going down the elevator there is a guy with his back turned who is perfect for a silent kill with the crossbow. Now i had saved my game before this cause of the group of guys after him for the RC bomb car chevo. After trying numerous times and continuously reloading my save this achievement popped. I had killed 2 people earlier so it was right at 8 times that I got this. So apparently reloading saves works for these types of achievements.