Recruit a GSL gang member and get in a small vehicle (preferably a quadbike) and drive to the building by the freeway with the escalators in los santos, drive up the escalators with the quad and get off and command your homie to stay by the enter doors. Hop back on your quadbike and park it in front of the doors. Go inside and press Y, CJ will run toward his bike and if you did it correctly, should be outside in the underworld, with your homie standing right there. Walk up to him and you will go right through him. You can shoot him and the bullets will go right though him, same with rockets and flamethrowers, ect. But if you do anything to harm him, and go back inside and then out again to be back in the real world, he will be dead.