The Battle Mage subclass can be unlocked by using a Tome of Mastery after defeating the second boss (Tengu).
When fighting with a wand in his hands, the _Battlemage_ inflicts additional damage depending on the current number of charges. Every successful hit restores 1 charge to this wand.
Using an Armor Kit on any armor will transform it into the Battle Mage Armor, which is specifically designed for this subclass.