Whenever you cast a spell that deals hit point damage, you deal extra damage to creatures made of metal or wearing medium or heavy metal armor. Affected creatures take additional damage equal to 1/2 your wizard level. This bonus damage is not increased by Empower Spell or similar effects. This bonus only applies once to a spell, not once per missile or ray, and cannot be split between multiple missiles or rays. The damage is of the same type as the spell.
At 20th level, whenever you cast a spell on a creature made of metal or wearing medium or heavy armor, you can roll twice to penetrate the creature's spell resistance and take the better result.