1. Host a game yourself.
2. Add an AI opponent.
3. Choose Wood Elves for yourself and whatever for your AI opponent.
4. Fill your team with Waywatchers (archer unit) and max their rank to gold 3 or rank 9. (you need to go to settings > funds > custom > 100000 for this to work)
5. Add just 1 general to your AI opponent.
6. Launch the battle. Just hastily click start battle and put the speed of the game to the max. Attack.
7. Enjoy your achievement.