This achievement requires you to complete the entire Doom 3 campaign on the Nightmare difficulty setting.
In this mode, you will begin the game with the Soul Cube already in your weapon inventory in Mars City and you get to keep it throughout the game. You will also slowly lose 5 HP every few seconds until you reach 25 HP, in which it will stay until you gain more health.
The best technique to keep using throughout Nightmare is to first of all, save constantly. Make sure you save before you turn every corner.
Another helpful hint is to use the Soul Cube on harder enemies since it is a one hit kill on everything except bosses and then use smaller, easier enemies to charge it back up again.
Ammo should not be a problem and make sure to use your weapons wisely. Use the Pistol or Fists for zombies, Shotgun or Machine Gun for Z-Secs, Chaingun and so on for larger enemies, etc. The Shotgun and Super Shotgun in this game is amazing at close range, and will usually one shot kill an Imp and similar enemies at point blank range. More often than not you can completely gib Zombies.
Attacks from certain enemies can be avoided, for instance the Imp’s fireball. Once it is thrown, simply crouch and you will duck right under it.
Focus your fire on the toughest enemies first. Perfect examples are the Arch-Vile later in the game. He won’t be a problem really, but for Nightmare you will definitely want to smoke him with the Soul Cube as soon as possible. Prioritize your enemy kills.