Description of Achievement: Score "C" rank or higher on any mission in any difficulty mode
Earn this in the Crimson Dragon Walkthrough

How to Earn this Achievement:

This is a very simple achievement that can be unlocked in the training mission of single player. Throughout the mission, try to avoid being hit and kill as many enemies as possible. Based on this, you will receive a ranking at the end of the level. As long as it is above C (they are based like school grades, so F,D,C,B,A, etc.), then the achievement will pop. The training mission is incredibly simple and easy so you shouldn't have a problem with attaining a C rank or above. Worst case, replay the mission. If for some reason it doesn't pop on the training mission (it did for me and one of my friends), simply play the first level which is also pretty easy. And remember this can be done on ANY difficulty.

P.S. While going for this achievement following this guide (so the training mission), you will unlock the "Act of Courage" achievement as well.