At the part where you get to the top of the tower and meet up with Johnson, you can get him down by either shooting at him or jumping on a ledge over the door, then to Johnson's ledge. When you get him down, kill him once and take his Spartan Laser. When he gets up, he should follow you until the door to the Oracle battle. When you get to the last door, shoot him with the Spartan Laser to kill him. Before he gets up again, run through the door. If done correctly, after the Oracle battle he will be in the location you killed him, but alive again. Note: After the Oracle battle, Johnson will not get back up if he dies.