You must beat this mission on Professional for the achievement. The mission is called "Kiev" and is pretty easy if done with a partner online thorugh co-op. The only semi-difficult part people seem to have trouble with is the back to back. For the back to back, try to stay on the sides it starts you on, don't keep turning around because that will confuse you. Stay on your side and take out the enemies quickly. Other than this the level is pretty easy. I have head reports of people putting the difficulty to recruit for the back to back, than afterwards switching back to Professional and still getting the achievement but I have not tried this so I can not confirm it.