Complete Normal mode(automaticallythe only difficulty till you beat normal mode) to unlock Hard mode at bottom of the main menu where you choose what you want to do, put it on hard, complete hardmode as fast as you can only get special weapons items if you need them, then you unlock easy which you shoul already have the 'Buster Max' which is self-explainable what it does you may or may not have to equip it on your buster.