0 0 0Ending on Normal difficulty will also open up the achievement for easy difficulty.Here is 100% complete guide to all achievement/collectibles and full game:
0 0 0My advice here is to initially play the game on Normal your first play through, and then go straight to Nightmare for your second. On Normal, you will have a higher chance and easier time to gain the largest amount of achievements. You should have no issue getting through the game on this difficulty.