Russian Martial Arts approach this from a different perspective. Rather than trying to make the body “harder”, they start by learning to absorb blows with incrementally increasing power. “Absorption” is done via breathing, relaxing the body and allowing it to move in the direction of the force.
Most untrained people react to a blow to the stomach by tensing their abdomen and holding their breath - according to RMA this is the wrong thing to do, as it inhibits movement of the body allowing the force to be dissipated, and in effect holds the internal organs in place so they wear the full impact. holding the breath further inhibits movement to allow dissipation of the force.
If you exhale and consciously relax with the blow, while moving with it, you can dissipate the force over a short distance, and the internal organs can move around the impact resulting in less trauma.
Anyone who has done BJJ or sub wrestling for a while will remember what it felt like being put in a decent knee-on-stomach position for the first time. It feels like you’re going to get squashed - some novices even tap at this point. By relaxing and not fighting it you can plan your defense or escape from this position more effectively. And you can take a LOT of weight.
RMA don’t try to make themselves invulnerable to attacks, rather to remove the overreaction resulting from the fear of being hit. That allows them then to respond more effectively to an oncoming shot, and makes the training more realistic by allowing the trainees to work with powerful, accurate strikes.
This approach does work with face shots to some degree, you can lessen the impact by turning or rolling the head with the shot, the basis of “shedding”, or “rolling with the punches”. I’m sceptical about whether doing anything other than VERY light drilling with head shots is a good idea and feel this is better learnt during sparring where you will get hit anyway.
It also comes down to protecting soft targets, e.g. better to take a punch on the crown of the head or glove rather than the jaw, and probably better the jaw rather than the throat or the temple. It’s always better to not be there, but it’s a luxury. IMHO it’s better to be able to deal with the worst case, then anything else is a bonus.
If you do any hard sparring or fighting, you WILL get hit - I was sparring last night and had to “absorb” a few good head shots. we all have to deal with it one way or another, RMA’s way is one, iron shirt or golden bell another.