Open the missions in Dromed, and change all the AI's awareness and aggressiveness to ultra sensitive (imagine all your AI with the skills of Haunts or assassins). You can also add more AI here and there, and remove healing potions, equipment, etc.
If I'm not mistaken, you can also change their speed. I remember playing the Ultimate Difficulty mod, and the Zombies were sped up considerably, which made them extremely dangerous. The mod no longer works, because it wasn't configured for Newdark, so you'd have to revert back to the old Ddfix Thief to use it.