How do you vs your shadow in custom robo arena

After defeating Eddie and hadron you will go back home after the prologue there , go to the club house in school and you will be in full black and be standing outside after defeating you , you will get an illegal robot then you have to get the dark commander band and be aloud in the underground where you can purchase illegal items (in the city where you fight solax ) in one of the houses talk to the man and some stairs will appear in front of a bed that is the underground.