Battle mode will automatically be saved to the memory card and be selectable at the Main menu when the game has been successfully completed on any difficulty setting with any ranking.
First Person View Option in Battle Mode:
To be able to enable the option to play Battle mode in first person view you
must first successfully complete the game on the Easy or Normal difficulty
settings with any ranking.
To unlock the Rocket Launcher in a new game you must first finish a game in under four and a half hours with an A ranking.