Updated for 1.22
Quen makes most difficulties easy mode because it can make Geralt ignore one blow entirely. This balances Quen to be not so overpowered. Current settings are that Quen only absorbs 50% of damage received, but takes double damage before it breaks. Alternate Quen heals only 75%. This has been tested with Vanilla Deathmarch and seems allright. I aim to use this with Brutal Combat, which will soon get an update. Let me know if it needs tweaking for both Vanilla and/or Brutal Combat. The balance may be cool for both with and without that mod.
I recommend using this tweak which will make Witcher 3 much harder - with:
Stamina n Skills
Brutal and Realistic Combat