The Sniper subclass can be unlocked by using a Tome of Mastery after defeating the second boss (Tengu).
_Snipers_ completely ignore enemy damage reduction (DR) when using a ranged weapon. This makes them particularly effective against heavily armored enemies.
Using an Armor Kit on any armor will transform it into the Sniper Armor, which is specifically designed for this subclass.
The Sniper's ability is implemented by overriding the Hero's ignoreDr() method to return true when the Hero has the Sniper subclass and is using a ranged weapon (rangedWeapon.valid() && subClass == HeroSubClass.SNIPER).