Vulnerability is a term used to describe a characteristic of Elementals, who take double damage from certain spells. Interestingly, Air and Earth Elementals are vulnerable to spells of their own schools of magic, but Fire and Water elementals are vulnerable to spells from the opposite school. Vulnerability could be considered an opposite to immunity.
Air and Storm Elementals are vulnerable to Armageddon, Lightning Bolt, Chain Lightning and Titan's Lightning Bolt.
Water and Ice Elementals are vulnerable to Armageddon, Fireball and Inferno.
Fire and Energy Elementals are vulnerable to Ice Bolt and Frost Ring.
Earth and Magma Elementals are vulnerable to Meteor Shower.