Originally Posted by Rurik
While the overall difficulty of the game isn't hard, it did have some frustrating moments (e.g. the heliocarrier mission). I found an easy way to get through the missions while still receiving the achievement.
The key is that the game only checks your difficulty when you complete the level to denote what level you placed at.
1) For the troublesome levels, play through on easy.
2) When you are in the last stage of the game (received last checkpoint), press Start and Save Checkpoint.
3) Press Start and Return to HQ
4) Press Start and change difficulty to formidable
5) Press Start to save difficulty
6) Quit to Title Screen
7) From Title, continue the campaign and load the last checkpoint
You will then start on the last checkpoint as formidable.
I just tried this on missions 7 and 8 and did receive the achievement. On mission 7 I progressed to the grid area on easy. I came back as formidable and took out the turrets and hacked the three computers to finish it. On mission 8 I played as Easy until the very last War Machine stage where you shoot the head. I quick and came back as formidable, destroyed the head, and received my Normal and Formidable achievements.
I agree heliocarrier mission was kind of retarded. Thanks for the tip this is the only one I've left.