Battlefield Hardline

C Lo eLoRiGiNaL is exactly right, the best way to get kills with the throwing knife is on Blood Money. I tend to do better getting the kills defending the base vault though, as opposed to the Money Pile, but it really depends on the map.

Good maps to do this on are ones where the base vault or money is inside a building and you can get high (like the Docks or Hollywood Heights). I've gotten most of my kills so far by hiding up high and firing down when I hear the "vault is being raided" prompt or see the money pile light up.

Finally, the throw action is very long, so to ensure a quick kill I like the hold the grenade button to "prime" the knife throw so you can kill someone in a quarter of the time it takes to make a full throw.

The easiest way I found to get this achievement was to play Blood Money since it is a central objective which attracts everyone and they will obviously be crowding. Depending on whether you're playing with a squad or not, you might want to lay down some cameras (as a professional) facing the money pile so as to know when to pounce on unsuspecting enemies with the knife, which is silent, so if you catch more than one enemy with their back to you, you can quickly grab a couple of kills. Any other tactics anyone might think work faster, just leave a comment and I'll be sure to edit those in.