The achievement is broken in the current version of the game but you can still earn it if you revert back to a previous version of the game.
In your Steam library, go to the properties of the game, go to the betas tab and choose “patch43 - Patch 4.3”.
In the map editor, create a scenario with 8 players.
Place 1 Archer for Player 2 (Red), 3 (Green), 4 (Yellow), 5 (Cyan), 6 (Purple), 7 (Gray) and 8 (Orange) and surround them with 56 Arbalests for Player 1 (Blue).
Save your scenario and leave the map editor.
Start a standard single player game.
Select your scenario, set the difficulty to moderate and start the game.
1x Archer for Player 2 (Red), 3 (Green), 4 (Yellow), 5 (Cyan), 6 (Purple), 7 (Gray) and 8 (Orange)
and 56x Arbalest for Player 1 (Blue)