Throwing knives are fairly common once you progress into the story. You can either find them or buy them from a weapons dealer. Something that you must do with throwing knives is hold down the . Doing so will allow you to throw the knives harder and perform and instant kill. If you just throw the knife without holding down the trigger you will only injure and not kill your enemy.
Killing 10 enemies shouldn't be too difficult, but always do it silently and stay in the shadows. You will have to do this as you progress through the game, you cannot reload checkpoints to earn these kills. You can also use the to stab people with the same knife, if you run out of knives to throw you can alternatively stab your enemies.
These have to be 10 kills as you progress, you cannot reload checkpoints.