***This achievement requires "The Crow" DLC***
The Battlefield is an alternate area that can be accessed from the Bayou (which is also an alternate area).
The Bayou can be accessed from the Crypt using the Teleporter Rune (look for a ledge with a teleporter pad on it), then the Battlefield can be accessed from the Bayou using the Summoning Rune (look for a ledge with nothing on it or under it, approach for a rock bridge to spawn).