There are some difficulty tweaks around here and I've tried them all. However, I may have some different approach than those. I'm not saying my way is better but it just more suitable with my taste.

A mod that I tried only add superb difficulty on lower level and slowing your level gain.
However, I think the problem with Deadfire is on the higher levels, where every attack of your enemies doesn't matter and you are one-shotting them, with almost always crits.

Also, I prefer to have normal level progression since I will get more tools to deal with enemies, rather than slower leveling. However, what I want is: the more tools I have, the harder enemies I will find.

Introducing New Penetration Rules
I think I found out the main problem where it gets easier on higher level: your health per level is so much higher than enemy weapon damage. Vanilla penetration rules only have up to 2 penetration ratio multiplier (the gap between penetration and armor) which is translated to +30% damage.

UPDATE FOR 2.0 Game Version
This is harder than I thought but finally this 1.5 version is the final version. This is the version that I use myself for finishing BoW DLC too. 

I also updated this to work with 2.0 game version. There is a ReadMe (in the Docs tab) if you want to take a look at the details for the final version. 

Now, I think I could get constant challenge even at the end of Main Games or even BoW expansion.

How to Install?
As other mods, put the extracted folder on the 'override' folder. For example, this is the complete address from mine:

C:\Program Files (x86)\Steam\steamapps\common\Pillars of Eternity II\PillarsOfEternityII_Data\override\Higher_is_always_harder

Credits:
Obsidian: It doesn't matter how many new bugs you introduce each time you release new patch, you will still be one of my fav devs!
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