I am writing this as it seems there is a lot of confusion between contracts and Missions.
for this achievement and the version for the other 2 divisions you need to complete the Missions, you will need to complete contracts but you dont need to do them all. If you tumble upon one that is too hard or time consuming just go ahead and discard it and try for an easier one
Each of the 5 main islands has one Mission for each division These missions are not available off the bat on most islands. you can check on the island management under the reputation section how much reputation on each division you need to build up in order to unlock each mission. The reputation bar is increased by completing contracts. (you can speed the process by requesting contracts manually from the contracts section under the island management Menu)
Once you have completed the 5 missions for the respective division you will get the achievement.
*In case you fail a mission just go under the mission section on island management and select the mission you want to restart and go from there. You dont need to restart the Island.