Enter your PDA, the select 'Objectives'. Hold L and press B, A, X, Y. Exit the PDA to advance to the next level.
Complete the game once under any difficulty setting. When you start on Nightmare Mode. you will start with the Soul Cube.
Pumped Up Soul Cube
After you acquire the Soul Cube, equip it and wait about one to two minutes. It will reveal its blades and spin for a very short time.
If you can avoid it, try not to use weapons like the shotgun or others that have difficult to find ammunition on the weaker zombies. The flashlight (if you are fast enough) or just the pistol usually will suffice.
Enter your PDA, then select "Objectives". Hold L and press B, A, X, Y. Exit the PDA to advance to the next checkpoint.
After you have found the lost scientist and are returning to the main building, find your way back to the bathrooms. Stand in front of the mirror with your pistol then cycle through until your shotgun then get very close.
Classic Doom reference
When you get to Mars City, go to the kitchen. Look closely at the video game in the back to see a classic Doom screen while you punch the turkey.
Be prepared to die a few times. Beware of the edge of the room, as you can fall off. Also, do not hide behind the pillars for too long. They will open up and arc electricity periodically. When you walk into the room, a giant bay door at the opposite end of the room will open, revealing the "new and improved" Sarge. As soon as that door opens a crack, start lobbing rockets in there. After you score about six good hits without being splattered by his BFG, break out the Soul Cube (that you should have charged before entering the room) and let him have it. It should kill him and heal you. Pick up the armor and ammunition around the room before continuing.
Defeating the final Boss
When you each the end Boss, do not bother wasting your ammunition on him. Just stay away from him. Kill the Imps and other demons and use the Soul Cube on him. If you are play in co-op mode, shoot him. Use the Soul Cube on him about five or six times. On the last time you will see an intermission sequence of him dying.
Defeating the Guardian Of The Soul Cube
When fighting the Guardian Of The Soul Cube in hell, do not bother shooting his body. This is a waste of precious ammunition. Instead, blast his "searchers" with something with good coverage like the chaingun, and he will have to spawn more. When he does, a blue light will appear over his body. Shoot it with the rocket launcher. Do this a few times and he will die easily. Also, look in the room around the columns for ammunition, health, etc.
Doom Ultimate and Doom 2: Cheat mode
Enter one of the following codes during game play. A sound will confirm correct code entry. Repeat the code to disable its effect.
Doom Ultimate And Doom 2: Toggle God And Degreelessness Mode
Hold L and press A(4).
Doom Ultimate And Doom 2: All Weapons, Health, Keys, And Armor
Hold L and press B(4).
Doom Ultimate And Doom 2: Temporary Invincibility
Hold L and press X(4).
Doom Ultimate And Doom 2: Full Map
Hold L and press Y(4).
Doom Ultimate And Doom 2: Berserk Mode
Hold L and press X(3), Y.
Doom Ultimate And Doom 2: End Level
Hold L and press A(2), B(2).
Doom Ultimate And Doom 2: Chainsaw
Hold L and press B(2), A(2).
Doom Ultimate And Doom 2: Invisibility
Hold L and press X(2), Y(2)
Doom Ultimate And Doom 2: Light Amplification Goggles
Hold L and press Y(3), X.
Doom Ultimate And Doom 2: Radiation Suit
Hold L and press X, Y(3).
If the Imp is jumping at you, blast it while it is in the air with the shotgun to kill it in one shot. This is slightly difficult to time.
Killing Teleporting Demons
When a demon teleports in (generally signaled by whisperings, a red tint throughout the screen, and always red lightning in the location where they will appear), you will have a split second before they are able to attack. Take a good weapon (for example, a shotgun or chainsaw for imps and wraiths) and stand close by. For powerful creatures, such as Pinkys, use a ranged weapon and stand at a safe distance. Remember to save the game often.