NPCs and sentry turrets are dealing double the intended damage to players in the scenario compared to what they are set for in the scenario's configuration files.
This means if we intend a NPC to deal 50 damage to a player based on a certain armor value/difficulty level/etc, that NPC is actually doing 100 damage to the player.

For now I recommend players reduce their NPC difficulty level by 1, so if you're currently playing on hard reduce it to medium, if medium reduce it to easy. This will scale NPC damage to the intended levels for the scenario, but NPC effective health will be halved.

This is unlikely to be properly fixed until Reforged Eden 2 as it would require changes to potentially hundreds of NPC weapons in the config files.

Read on if you want to read the full boring math behind why this is the case.

Base damage:
This is the damage set in the config file. NPCs are configured in the EClassConfig file and each NPC is given a weapon item from ItemsConfig to use and that weapon determines how much base damage the NPC will deal before any modifiers.

Difficulty level:
The enemy difficulty level for your save game changes how much damage NPCs and sentry turrets deal to the player character.

• Easy: NPCs and sentry turrets deal 50% damage to players.
  
• Medium: NPCs and sentry turrets deal 100% damage to players.
  
• Hard: NPCs and sentry turrets deal 200% damage to players.

NPC level:
The level of the individual NPC further multiplies its damage, from a multiplier of 1x (no increase) for a level 1 NPC, up to 2.5x for a level 10 NPC.
This means a level 1 NPC on easy will deal 50% weapon damage to a player, and a level 10 NPC on hard will deal 500% weapon damage to a player.

Armor durability:
The damage reduction your armor provides is reduced by how damaged it is. Armor at half durability will only provide half its normal damage reduction.

Armor value:
This is where the math goes wrong in the scenario.
It was assumed that an armor value of 0 would provide no change in damage and therefor you would take 100% damage from NPC attacks if wearing no armor.
An armor value of 100 would reduce damage taken by 50%. And an armor value of 300 would reduce damage by 75%.

However what actually happens is that an armor value of 100 is what provides 0% protection, an armor value below 100 increases damage taken, having an armor value of 0 increases damage taken by 100%.

This means that the player is effectively taking double the damage that is set in the scenario's config files, no matter what armor value the player had.

For example:
If the player has 0 armor, we assumed they would take normal damage but they actually take double damage.
If the player has 100 armor, we assumed a damage reduction of 50% but actually they are taking full damage.
If they have 300 armor we assumed a damage reduction of 75% but they are actually only getting a 50% damage reduction.

Any adjustments for this will likely have to wait until Reforged Eden 2, as there are potentially hundreds of different NPC weapons that will need changing in the configs and it's not as simple as just increasing the armor on armor and boosts.
For now I recommend playing on easy or medium difficulty, adjusting as needed. NPCs will be fairly fast to die, but at least the damage you take will be more in line with what we intended.

I have been told that this is the intended damage scaling for the game, and that there have been no changes to this system for quite a long time, however please do not take this as an official statement.