In order to fight zero you must first aquire all of the biometal fragments and then get all their passwords so you are able to open the door in M area. Once you have gotten through M area and have reached the transporter save your game their and continue on to area N. When you enter go down the first ladder you see. Then drop all the way to the bottem and continue down the path on the right. You will come across to doors but just keep going until you raech the very last door.Go through the door and zero should be waiting in the middle of the room.