You can also unlock "Iron Body" with this method:
Find one of the many fire barrels in the game and push against it. Initially only JC's legs will get damaged regardless of which side of his body comes into contact with the barrel. Once both his legs are injured, turn sideways so that only a hand touches the barrel at a time and keep pushing against it. Eventually you will injure the arm as well, so just turn around and do it again for the other.
A perfect spot to do this is outside Tracer Tong's laboratory, since there's a barrel there and Tong has a medical bot in a room under the control room, where he stays. Alternatively, just save, do the thing and reload the save.