23 enemies in total and can be gained on multiple levels of course, but I've provided a list below and a level that they are easily available on:
DOOM 1Zombieman - Normal basic soldier - E1M1
Imp - Brown basic enemy - E1M1
Baron of Hell - Red Devil looking guy - E4M8
Spiderdemon - Giant Arachnotron boss at the end of the level -E4M8
Lost Soul - Small Flying Flaming Skull - E4M8
LEGACY OF RUSTTyrant - Huge devil looking guy that fires rockets, green waist - E2M7
Cyberdemon - Huge devil looking guy that fires rockets, red waist - E2M7
Shotgun Guy - Normal Soldier but red/black with shotgun- E2M6
Demon - Red Rhino type monster - E2M6
Shocktrooper - Normal Soldier with helmet firing plasma rifle - E2M6
Ghoul - Small Flying Skull but blue/yellowish effect - E2M6
Hell Knight - Brown Devil looking guy - E2M6
Cacodemon - Big Red Flying ball with one eye- E2M6
Mancubus - Big Chunky monster with cannons as hands - E2M6
Arch-Vile - Skinny Brown monster that sets you on fire - E2M6
Revenant - D00T - E2M6
Mindweaver - Small spider has normal red eyes - E1M6
Vassago - Black Devil demon that sets you on fire - E1M6
Banshee - Red Screaming Face, fairly rare. These pop behind the yellow key, so at the bottom of the spiral stairs take the right door into the lava and quickly follow it around to the right back on yourself to gain access to everything - E1M6
DOOM 2 Spectre - Same as the Red Rhino but almost invisible like a shadow - MAP 27
Heavy Weapon Dude - Normal Soldier but heavy duty, uses a chain gun and is red/yellow/black - MAP 27
Pain Elemental - Big Dark Flying ball with one eye, spawns lost souls - MAP 28
Arachnotron - Small Spider but with blue eyes - MAP 7
If you need more help with this you can follow my video guide below which starts at this achievement: