In my opinion, the game isn't really that hard on Nightmare, the game is really forgiving when it comes to health and ammo. You can always stock on bandages, and if you feel overwhelmed, most of the time all you need to do is step back and let your health recharge.
Learn how to use Wingsticks well and carry a bunch on you at all times, a well aimed Wingstick can kill most enemies with one hit and they have a chance to boomerang back to you to conserve ammo. Late in the game you can purchase an Advanced Wingstick schematic that unlocks an even better version of the weapon which, in my opinion, is one of the best in the game, if not the best.
Collect Desert Spores, these are rare collectibles, available at only certain locations in the game, which are required for making Apophis Infusion, which gives a permanent health bonus. Make sure you complete the side quest "Miracle Cure" which grants the schematic for this.
To upgrade your Defibrator, complete the optional quest "Finding Feltrite", the reward is an ugrade to the defib recharge time.
That's pretty much it. The game isn't really that hard, even on Nightmare, so just enjoy it.