Monster credits:

Zombie riflemen are Zsec from Realm667, submitted by Captain Toenail, sounds are ripped from EYE Divine Cybermancy

Grenade sounds ripped from the Sven co-op Insurgency addon, sounds made by New World Interactive, and R4to0 Converted them into .ogg format.
Additional grenade sounds ripped from Red Faction, Half Life 2, and EYE Divine Cybermancy
Grenade sprites ripped from the Melonades Addon for Insurgency, made by Melo, which in turn was an edit of the Freedoom armor bonus.

Shotgun guys are edits of the Zspecops also from Realm667 and submitted by Ghastly dragon, sounds are ripped from EYE divine cybermancy

The chaingunner is an edit of the Doom 2 chain gun guy by HorrorMovieGuy

Imps use sounds of goblins from Rune

Pinky demons, hell knights, barons of hell, Cacodemons, Spider Masterminds and Cyberdemons use various monster sounds from EYE Divine Cybermancy

Cacodemon charge shot windup sound taken from Unreal Tournament 2004

Pain Elementals use Netch calls from Morrowind, along with the Barney Gumble belch and a stock yell which I think is from a PeeWee movie.

Lost souls use cliffracer sounds, along with the same stock yell the Pain Elemental uses

Revenant homing shot sounds ripped from EYE Divine Cybermancy

Archviles use sounds lifted from Black Heart's voice lines MvC2 and the Pfhor from Marathon

Miscreants are voiced by Yholl

Arachnotron sounds are from the robot ninjas from MK3 (Cyrax, Sektor, Smoke)
Their charge shot comes from Unreal 2

Mancubi use edited voice clips of the Heavy from TF2

The sound that plays for when enemies are cut in two and explode from the blade beam from Combined_Arms comes from Megaman Zero

