Hint: Using barrels:
When entering any area that has several monsters of any kind look for green barrels containing toxic waste. Take out any gun and shoot the barrels to wipe out enemies quickly. Note: Standing too close to a barrel of green waste and shooting it can injure or kill you as well. Try to stand back when firing at those barrels. Also, it usually takes more than one shot to blow up a barrel from a distance.

Hint: Enemies:

Zombieman/Former Human soldier: They resemble your old friends, but now they just fire pistols at you. They drop clips for the chaingun and pistol when killed.

Shotgun Guy/Former Human Sergeant: 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.

Lost Soul: Floating, flaming skull.

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.

Baron Of Hell: Minotaur-like monster. They first appear at the end of Episode 1.