This achievement is unlocked during the 2nd mission of the second Germans campaign, "Teutonic Order".
In order to do this, you can ONLY get kills using ranged units. This means that you CANNOT use your hero (Konrad) during this phase of the mission, or you will void the achievement.
The setup I used was to put 4 Crossbowmen on the west entrance, 4 Crossbowmen on the north entrance and 1 Crossbowmen squad I used to hunt down catapults/rotate where needed. The AI can hold the east entrance on it's own. Make sure to upgrade your units to their full potential ie. armor, health upgrade and speed upgrade. Also make sure to build the towers on both the main castle and the villages, they will be very useful.
Provided you set up like this and keep your hero stowed in the back of the castle the achievement should unlock as soon as the enemies start retreating.
If you prefer, you could probably use archers to the same effect, but I unlocked it using crossbows exclusively.