Passive Breaking

It is common for a demonstration to feature the breaking of materials over the body. Such a practice is not an exercise in destructive force, but rather in the ability to withstand an attack. There are two general methods of this.
Striking the Material

The first method consists of resting a target, such as a cement block, on an appropriate body surface and having an assistant strike it with a sledgehammer.

I do not regard this as a meaningful practice of tameshiwari. I fail to see how it demonstrates any special skill or ability. It is illogical to think that the demonstrator is somehow withstanding the blow of the hammer. The impact is almost completely absorbed by the target material, which is why it breaks and you do not. Very little force reaches the body supporting the block. If the assistant takes the extra care to tap the block with a very short strike, even less force travels through to the body beneath.

A more impressive demonstration would be to place the body over a cement block and then breaking the block by striking the body. This would display the ability to withstand a breaking level force. I do not recommend the attempt of this with any material of significant sturdiness.
Striking the Body (with the material)

The second method consists of striking the body with the material to be broken. An example is having an assistant break a stick by striking it across oneď│ extended forearm. The total amount of impact absorbed by the body is greatly affected by the performance of the assistant. Such a practice may be considered a form of iron body training.

While this form can have certain benefits, I consider those benefits extremely limited. As with active breaking, the breaking material must be a challenge sufficient to require a conditioned surface. If the break can be performed without need of specific conditioning, it is not a unique skill.

Any individual with a fairly sturdy frame can withstand a certain level of impact. For this reason, the most convenient way to build a general resistance to attack is to build the body with muscular training such as weightlifting or isometrics. Certain, specific, areas may be further toughened with impact training such as the medicine ball drills utilized in boxing gyms.

This type of training will develop a satisfactory resistance to mediocre attacks and, possibly, even fierce attacks. There is no training that will allow one to absorb repeated, powerful attacks. Even in the best case, there is no training at all for strengthening the face, which is the primary target.