In the mission “The Moebius Factor” there is the new mission “Contract Saviors” which says <<Complete the mission by training only SCV’s Medivacs and Mercenarys on Normal Difficulty >>
I did this and was confused why i didnt get the achievement, and so i replayed it on normal and didnt made a single unit at all, since on that difficulty the mission is quickly possible with the units that you collect in the map alone…and still didnt get it. So i think this achievement is impossible to get no matter what you do.