Acquire 100% of the Stars
This achievement is easy, however will take some time to finish. Start by going to the Specialist HQ
, then play the skirmish for each specialist 3 times ( remember you must win). You will need to play a total of 30 skirmishes, 3 for each of the 10 specialist. Sadly you must complete each difficulty to unlock the next: Recruit, Regular, Veteran.
This video will show you how to unlock all 5 achievements in the specialist HQ.