Ultra-Nightmare difficulty is unlocked after you complete level 6. You have to start a new game to select the setting, you can not use your existing save.
The UAC is the first level of the game.
This difficulty means you will die within a few hits. Also armour and health pick-ups give you less than you normally get.
You have to keep moving pretty much all the time. Strafing as you go. If you get an opportunity to kill an enemy from a distance you should use your pistol to charge a shot. This helps minimise the enemy contact. Make sure of your grenades once you obtain it.
Expect to die a lot. It took me 50 minutes to get this trophy. Every run I learned more to help get me through. Such as enemies appearing behind me when I thought I had cleared it all. I then knew to wait around before proceeding.
Good luck