the same way wizards have basic ressist to reduce elemental resistance why not adding a rare that reduce total physical resistance? like 2% per grade, as an example:
g10=30% pierce physical resistance
physical resistance=white/blue/red ac+physical spell/skill shields
if oponent has 30% physical resistance his physical resistance will be reduce by 9% = 21%
if oponent has 50% physical resistance his physical resistance will be reduce by 15% =35%
if oponent has 70% physical resistance his physical resistance will be reduce by 21% =49%
if oponent has 90% physical resistance his physical resistance will be reduce by 27% =63%
numbers could be change.