I found the best place to get the stealth kills was on the "Gaining Influence" mission. This is on Disc 2, and the way point you are aiming for is Infiltrate Power Plant.
I put a save in before i entered the plant, (I had 2 stealth kills up to this point).
When you enter there will be a Gearhead whilstling whilst sweeping the floor, he has his back to you and his head bowed down slightly,and doesn't notice you when you enter. You just need to aim at his head and chalk up your kill, then turn around and leave through the door you just came through, and repeat till you have your 10 kills.
The game will give you a message saying that all mission progress will be lost, (and your stealth kill stats will reflect this), but the kills still count.