This could be: All weapon attacks made by a [class name] ignore resistance and immunity to bludgeoning, piercing, and slashing damage.
The idea of bypassing physical immunity but not resistance is kinda silly. This could be too weak, because many creatures that have immunity or resistance to physical, the resistance can be bypassed by magical weapons. Most melee PCs will have a magical weapon by level 10, as they start appearing at level 4, rendering a level 20 ability null and void.