Set up as follows:
Play with Friends (use an alt account or have a friend join).
Map: French Revolution
Type: 70% Domination
Dice: Balanced Blitz
Troop Placement: Manual
Blizzards will help, but are not mandatory.
THE PLAYER THAT WANTS THE ACHIEVEMENT NEEDS TO BE IN THE SECOND SEAT
Fire the lobby, and take one half of the map each.
When placing troops, have the player that needs the achievement place all of their extra troops in the corner; the player that is helping should place their extra troops on a territory that is open to the other player.
Once the first turn starts, place troops on the stack and wipe out the territories with one troop on until you hold 70% of the board - the other player will then get the achievement.