Vulnerability
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 Elementals and 
Storm Elementals are vulnerable to Armageddon, Lightning Bolt, Chain Lightning and Titan's Lightning Bolt.
Earth Elementals and 
Magma Elementals are vulnerable to Meteor Shower.
Fire Elementals and 
Energy Elementals are vulnerable to Ice Bolt and Frost Ring.
Water Elementals and 
Ice Elementals are vulnerable to Armageddon, Fireball and Inferno.