Score 2000 on the Mourningwood Fort mortar game.
First off you must score over 2,000 pts on the game for it to unlock.
You are required to purchase Mourningwood Fort to unlock the Mortar Range mini game. This requires the “Entrepreneur Pack” from the Road to Rule if you are going to play this solo but the best way to get this achievement is through co-op!
One of you does have to have the "Entrepreneur Pack" so let that person be the host. Then head to Mourningwood and start to play the mini game. Now all you have to do is get a combined score of 2,000 points instead of the full 2,000 solo. Much easier than doing it solo!