Special Operations Missions will become available as you complete all story missions in a zone. This achievement is for completing the Special Operation "Defend Albion Resources" located in the first zone of the game, "Kingston".
You can start a Special Operation by using the "Special Operation Terminal" in Albion, which is the one with the yellow globe, which can be found by moving into the center area and then turning left. Alternatively, you can teleport into the zone and use its specific Mission Node. In this case, you can teleport onto the top Battlefield of Kingston, using the Outpost "Grand Square", and it will be the closest node right in front of you.
The Mission itself will have you defending an objective for multiple waves of increasingly harder enemies that last 2 minutes each. The Achievement requires 7 waves completed which is also the minimum amount you need to just complete the mission. Afterward, you can either leave the mission or continue. If you do choose to continue, the mission can go on until waves 10, 14, 18, and 21, in which you have the option to leave after each milestone. Don't worry if you happen to lose after continuing, you will still receive the awards for completing the previous milestone waves of 7, 10, 14, and 18.