Battle Magic is unlocked during the Learning Battle Magic Quest, available at level 4. You can take this Quest from Storm Brigadier Rand in Vaalor War Academy or Crown-Legate Crimm in Sadar War Academy. Magiv can be used after equipping one of the three Class Amulets (Amulet of Berserk, Amulet of Paladin, Amulet of Witcher)