You can strengthen your hands and feet by useing a makiwara. While you can build your body up to take damage to a point, you cant strengthen your groin,temple's or solar plexus for example, the only way to protect these places is not get hit. As someotherguy said, better to learn to evade attacks, best block---dont be there. That goes for both the attack and the situation. When it comes to your friend hitting himself with a stick-----not good. You may want to ask some Kung Fu stylists about iron shirt methods.