This achievement is completed in the Teutonic/German campaign mission 2.
At the start of the mission move your hero as far south as possible, you don't want him accidentally killing things when you aren't watching as this will invalidate your run.
Next go to each of your villages and build your three watchtowers, as well as the three watchtowers for for main base. (you will lose these over the course or the siege - repair/rebuild where possible)
Then build 9x archer units. put 3x on the west wall south of the gate, 3x on the west wall north of the gate, and 3x on the north wall close to that gate.
purchase the archer upgrade, and the armour upgrade (Germans require an upgrade before you can increase armour) then deck your archers out with lvl3/gold armour - I'd prioritise the ones on the west wall, as they can get pushed by melee infantry. though you should have plenty of metal to armour all 9 fully.
Its pretty much sit back and watch until they bring up the siege weapons, when this happens you might need to micro control bringing units down to replenish (siege units wont injure, its either kill or not.). the fast fire archer ability is pretty handy, use where you can.
If all goes to plan, right when it looks like the enemy is about to overwhelm you, they should retreat, and the achievement should pop.
As with all other Ancestors Legacy achievements this con be completed on easy.