As far as I remember, PIERCING does not reduce DEFENCE by its percent, but instead subtracts PIERCING value from DEFENCE value. So, if the opponent has 50% DEFENCE and your fighter has 30% PIERCING, then the resulting damage output would be reduced by 20% (50%-30%). In other words, 30% of PIERCING ignores exactly 30% of opponent's damage reduction provided by DEFENCE, ARMOR, or special effects like Annie's BLUE SHIFT MA.