This Seeking Tech guide will explain how you can unlock the No Mercy trophy in Metaphor. You will need to end 50 battles without any of your party members taking any damage.
The best way to get this is to attack an enemy on the field until they get stunned. You can then enter the battle with the enemy already hurt and unable to take their turns.
You can also try targeting easier enemies and play at an easier difficulty setting.
At the end of the fight, you should see the Unscathed Triumph tag, which indicates that your party did not take any damage.