Computer player spells and their effects.

A complex topic. I know for sure that the AI won't cast elemental protection if there's no reason to. So, if a player has a chain lightning spell, the AI might cast air protection. And if the player's air magic is leveled up, the chances of it being cast increase.
In some cases, the AI prefers elemental protection over useful spells. In one battle, my hero had Meteor Shower, and the AI cast earth protection several times, even though it was an expert in water magic.