This guide was made using the default settings, including normal difficulty. And trust me, other than a few spots and most of the Flood encounters, the game is really easy on normal.
I refer to each level in two ways, just in case someone goes by one way and not the other. I'll name each section by the name in the theater, and by the name of the achievement after you beat a level. And if all else fails, just count the levels and you'll know which one you need.
Also, it's best if you read ahead of where you are just a bit. If you're afraid of spoilers, don't be, but I can't promise nothing. If I do spoil something for you, it will be when you get to that point in the guide. So if you can't handle that, I don't know what to say.