This is how it works:
Let’s assume you use axe attacks (which usually causes 200 damage)
First attack with Axe attack in a turn deals 3
200 damage to all enemies, and the Stoneskin meter jumps to 10%.
Second attack with axe in the same turn only deals 200* (100-10) = 180 damage. Stoneskin meter jumps to 20%.
Third attack in same turn with axes deals 160 damage, and Stoneskin meter jumps to 30%.
Forth attack in same turn with axes deals 140 damage, and Stoneskin meter jumps to 40%.
Fifth attack in same turn with axes deals 120 damage, and Stoneskin meter reaches the 50% maximum.
Amy further attacks in the same turn deals half damage.
At the end of your turn all ailments are cleansed from the enemies, and Stoneskin buff is actives for 1 attack, and Stoneskin meter drops to 0%.
So the first attack to any enemies deals only 1 damage.
This Stoneskin buff can be applied not just to bosses, but to normal monsters too.
Stoneskin meter is reduced with 10% if you do not use any specifial skills or battle items during a turn.
Let’s assume it is at 30%. If you do not use any skills or items for 3 turns it returns to 0%.