To get it u need hammer V2 from Raul Get all V2 weapons: katana , void sword , terraformer , fist of fury . All need to be V2 to enter the door Hammer you get it from mega dummy.
tbh i already have the weapon i also got v2 for about 4 weapons but u did give me an idea of making a tutorial how to get it my own Edit: the hammer is also called "Dummy Destroyer" btw