game level completly debalanced

The difficulty setting is a misnomer. The only difference it makes to "difficulty" is changing the balance of damage and chance for rage mode.

0. Scavenger
Skill - Easiest
Description - Easiest setting designed for noobs.
% Damage by Zombie - 50
% Damage by Player - 200
% Chance for zeds to rage - 0

1. Adventurer
Skill - Easy
Description - Not just a new recruit any more.
% Damage by Zombie - 75
% Damage by Player - 150
% Chance for zeds to rage - 20

2. Nomad
Skill - Normal
Description - Designed for experienced FPS players.
% Damage by Zombie - 100
% Damage by Player - 100
% Chance for zeds to rage - 40

3. Warrior
Skill - Hard
Description - Now you're starting to impress.
% Damage by Zombie - 150
% Damage by Player - 83
% Chance for zeds to rage - 60

4. Survivalist
Skill - Harder
Description - Kids don't try this at home!
% Damage by Zombie - 200
% Damage by Player - 66
% Chance for zeds to rage - 80

5. Insane
Skill - Hardest
Description - You're one brave Mother!
% Damage by Zombie - 250
% Damage by Player - 50
% Chance for zeds to rage - 100

And keep in mind, that this has zero effect on your gamestage.
Also note that changing your game day length will allow you to increase your gamestage exponentially before the horde night.

What you remember about a13, is those same damage curves, with no rage mode, and the difficulty of the horde was based solely on what day it was. That hasn't been the case for several years now.