[@Ellri] Depends on the spell. For example in the battle of Ralda most of the fireballs were extinguished before they even reached their target. Other methods could be using a magical barricade (taught under the destruction school as a form of defense) which could be used to block against most spells (but useless against actual physical weapons). Mages may erect a barrier of ice or stone if they wish to protect themselves from more powerful spells or mundane weaponry though this takes more skill then a simple magical barrier.