This response is a month old, but is here in case anyone else runs into this problem, especially using the trainer.
First, you need to take the miners that appear on the north part of the map, and send them down to each land bridge. Right click the ends of the land bridge and they will begin "destroying" (building) it.
Second, if you have a trainer and invincibility is enabled, you cannot complete the mission. You must start the mission without invincibility, and keep it disabled. The land bridges count as your own buildings, and you cannot complete destroying them if they take no damage.
Hope this helps.
Of course it does sir, and you are a mad pro genius!!!