Incorporeal

Creatures with the incorporeal condition do not have a physical body. Incorporeal creatures are immune to all nonmagical attack forms. Incorporeal creatures take half damage (50%) from magic weapons, spells, spell-like effects, and supernatural effects. Incorporeal creatures take full damage from other incorporeal creatures and effects, as well as all force effects.

Marius, Scholai Armsmaster Says: Incorporeal monsters may ignore armor and shields, but magical protections like Mage Armor and Protection from Evil function normally.  These first level spells make for excellent and affordable potions, wands, and scrolls for the aspiring ghost-hunter.