Try to drive your heel into the top of your attacker’s foot. Even if you’re not very big, you can still break their arch, ankle, or toes by doing this. Most of the time they’re not expecting it, and if done well, it can cripple them. You can either run away at this point, or take advantage of the opening you’ve created to deal an additional disabling blow.
If you’re grabbed from behind as in example 3, aim your heel for your attacker’s leg anywhere at or below their knee. Even if you don’t land a direct hit on their leg, you can still get to their ankles and feet.
Body blows are much harder to take than head shots. If you’ve got an opening below the neck, use it. Hit it with anything you’ve got.