This achievement is for getting 50 soldier and 5 dog silent kills.
As there are a number of other perks linked to silent kills, the 50 soldier tally shouldn't be too bad. The problem I had was finding the dogs!!
The best place I found was level 4 (the prison), when you infiltrate the police building. At the start, after getting the laser cutting gun you have to jump onto the roof the other side and open a window. There will be a guard walking away from you so quickly knife him. The crouch and walk forward and into the room. Another guard will walk round but won't see you, so creep up and knife him. Don't worry about the other guard, instead go straight forward into the room across - this has a dog asleep so knife away.
If you then select restart checkpoint, you will start just before the window opening bit, but keep you 2 soldier stealth kills and your dog kill.
Lil Miss Cherry has kindly emailed a link (see below) with a video guide in respect of this method, and credit to her for this!