Syl, premonition works exactly like stoneskin and greater stoneskin (well, it's stronger than either now). Unless you have a +5 EB on your weapon the first 30 points of damage dealt per attack are absorbed, with a total of 10 points of damage per caster level being absorbed before the spell fades. So a level 20 mage that casts premonition will absorb the first 30 points of damage from every attack until they have absorbed 10 * 20 = 200 points of damage. If the person attacking them has a +5 EB on their weapon the spell is bypassed and absorbs nothing at all.