I had two methods of getting this achievement. While being an archer have the dagger as your secondary weapon (it's default) and when someone gets too close, just fight him with blocking and attacking with the dagger.
Another method which is slightly easier that I used is the be the man at arms class, with a shield equipped, when the spawn change weapons to your dagger and you can use the shield and the dagger together to make blocking a bit easier.
You will need to get 75 kills with a dagger after unlocking the second dagger, so that is a total of 100 dagger kills.
Any suggestions for this solution just comment. :D