Normal Difficulty ( renamed to Very Challenging )
Is very challenging by design, game was created with this difficulty as default, hence it called "normal".
You will need to sprint around fast, often prioritizing survival over shooting
switch weapons more instead of reloading machine gun midfight
use Slam Away strategically (kills with Slam Away reload all guns)
use pickups strategically (when you have 100+ HP/Armor you get 30% from pickup)
But to make it not so frustrating there's plenty of checkpoints so you don't loose much time when dying.
Lower Difficulties
You can scale it down all the way to 5% of incoming damage and enemies run speed to 50%
Issues I know of
Slow loading time, especially on lower end CPUs - this will be improved ( UPD. - fixed )
Crashes, especially on lower end CPUs - weaker CPUs often crash apparently, looking into that also ( UPD. - fixed one cause of crash, not sure is there any other causes)
Update notes
fixed that sometimes difficulty incoming damage multiplier didn't work
adjusted difficulty presets - all levels below normal now have much lesser values