Using sounds to find monsters
Listen for weird sounds, like somebody breathing very hard during the game. Those sounds are the brown monsters that throw fireballs. When the sounds stop, that means you have killed them all. However, there still may be normal enemies with guns and shotguns or pig monsters.
The snorting sounds in the game are coming from the pigs. Once the sounds stop, that means you have killed all the pigs.
Cheat codes cannot be used in nightmare mode. It is just like the regular game, except the monsters are stronger and harder to kill. Enemies will keep appearing non-stop on each level -- you have to try and get to the end before more appear.
Former Soldiers: They resemble your old friends, but now they just fire pistols at you. They drop clips for the chaingun and pistol when killed.
Glitch: Living death
Former Sergeants: Meaner and tougher. They carry shotguns and do more damage then soldiers. They drop shotguns when killed.
Imp: This creature throws fireballs and takes more bullets to kill.
Demon: Appear in most areas and can rip your head off if you are not careful. If you see more of them, use the chaingun.
Cacodemon: These creatures are ugly and have one big mouth. If they see you, they will fire lightning balls from their mouth. Use your rockets.
Mancubus: You have to use more then a chaingun to kill them. Use ammo with caution.
Arachnotron: Fires plasma energy at you. Use your rifle.
Spider Demon: The ultimate enemy. Has a super chaingun mounted on her body. Use your plasma rifle or rocket launcher.
Allow an enemy to kill you. Once you have a bloody face and you are on the ground, enable the "God mode" code. Your character will come alive again, but will not be able to change guns, move, or shoot.