Da Lukinator has everything down to a tee, just wanted to mention some of the differences between both difficulties.
1) You'll notice in regular mode that as you progress through the chapters, more roads get blocked off (by military, walls, debris etc..). In nightmare, right off the bat most of those roads will already be blocked off and it will progressively get worse as you move along through the story.
2) In regular mode, there's not a crazy amount of cars scattered around the city (if you didn't notice). If you've ever said "why the hell is there no vehicles when I need one?"… In nightmare you'll be saying "why the hell is there no vehicles?"… So again, make sure the attributes for "Mechanic
" is maxed, because vehicles are very limited.
3) Like Da Lukinator mentioned.. SAVE EVERYWHERE AND ANYWHERE
.. You can't save from the menu, and you also don't save from completing chapters. When ever you see a porta-potty OR you're in a safe house.. spare yourself the frustration and take a nice piss (A.K.A save lol). You can lose a lot of game progress if you die or the host disconnects from the internet randomly.
4) Although you don't have to do all the Psychopath (boss) battles, there are some mandatory ones. They have the same amount of health, but take a lot less damage from your weapons. I recommend using the super-combo weapon called "Elemental Staff
". This gun is super savvy and hands down my favourite combo. (You can switch through the elements of the staff simply by pressing (Y)".
5) Also quoting Da Lukinator, ignore every side mission. This is because you'll notice the days are a lot quicker. Side missions have nothing to do with the main storyline (thus the title side mission).
Having a second person is EXTREMELY helpful if you have anybody willing to speed run through the story with you.
(SPOILER FOR THE END OF NIGHTMARE MODE)