This post is to help you tweak different factors in order to make the game easier/harder. There are a few things that should always be constant as the mod is specifically balanced around it:

• Easy soviet AI
  
• Zombie spawn rates
  
• 200 population cap
  
• 2x resources

The easy AI is required as it prevents retreating behavior among other changes. As of 1.12 you may use Expert AI for zombies as well. This AI is less aggressive and tends to gather units for horde attacks. In theory it's 'easier' because it allows players more time to prepare defenses.

Zombies will always spawn the same amount of units regardless of difficulty or income. Each zombie squad is worth 8 pop cap. Pop cap restrictions are ignored for some special zombies but they still contribute towards the cap. For example if the pop cap was 200/200 and a normal zombie and charger tried to spawn, only the charger would spawn and the pop cap would end up being 208/200. Turned and carcass do not contribute to pop cap.

The greatest factor for difficulty is the map. The amount of choke points determines how concentrated or thin your army will have to be in order to hold a line. Maps with lots of blind spots tend to be more difficult as you wont have the advantage of range. Easily holdable territory nodes will also determine if you'll have enough income late game to defend against hordes.

Open maps are a double edged sword. Whilst they allow you to make full use of range, it may spread your forces too thin depending on the amount of area you have to cover. More importantly, open maps allow for better maneuvering of tanks. Small maps are naturally easier because the short travel distances lead to faster matches and thus less chances of reaching higher tier zombies. Maps with only 1 choke point are not recommended at all. It will often lead to both sides being unable to push past the choke.

The AI is stupid. Although zombies are supposed to be stupid, non-zombie AI wont be able to make smart decisions and will often fall past the 30 minute mark. Using AI teammates is almost always worse than having human players. The only thing better that the AI does is pushing early game because of their increased resource income.

As of 1.12 it is recommended to use Standard or Hard AI for non-zombies. Both AI have 10x munitions. Standard has 3x fuel and manpower which will put them on par with a zombie AI for around 30 minutes. Hard has 4x manpower and 10x fuel which will make them stronger than a zombie AI because of the amount of tanks it deploys. Like the zombie AI, the performance of the non-zombie AI is highly map dependent.

It is not recommended to use Easy or Expert non-zombie AI. Although they get 10x for all resources they will just run all of their units into zombies since it's designed to enter melee range.

Because zombie spawn rates are always constant you could do something like having a match where it's normal resources, 200 popcap, 2 humans vs 1 zombie which in theory would be around the same. Of course there may be a slight disadvantage in this because you'll have to spend resources teching up.

• Increase income amount (Win conditions)
  
• Maps with few choke points
  
• Maps with few blind spots
  
• Maps with bridges
  
• Smaller maps (2v2)
  
• Decreasing AI to human player ratio

• Decrease income amount (Win conditions)
  
• Maps with lots of blind spots
  
• Larger maps (3v3 or 4v4)
  
• Increasing AI to human player ratio