I got this achievement on the Snowblind mission. When you are forced to go from house to house warming up, before you take off to the next house sneak a peek at where the next one or two opponents will be. Let them walk around the house and sprint down and knife them one by one. I was able to get all 10 knife kills on this one mission, but I was on the medium difficulty. I'm sure it might be a little tougher for those who are playing on HARD.