Less of a guide and more of a note. If you do the apocalyptic difficulty which opens up after completing the game the first time then you will also get credit for doing all of these missions as well. You don't have to play on hard at all.
I suggest playing on casual for your first play and get everything that you can possibly get there, including all character upgrades. This still will all carry over to later difficulties. Then just play through on apocalyptic and bypass this difficulty altogether.