It also really depends upon the amount of damage the unit sustains at once before the handler, or script can take effect. If the damage instantly inflicts total damage, such as an explosion, the handler will not likely be quick enough to reset the damage.
Hit handlers are quick but a fast looping script is quicker. Since it is continuously running and seeing as how you would only want it for testing or cinema, you wouldn't need it for long or in broad capacity, so its effect on CPU would be minimum.
Just have a looping script constantly setting the damage of the player/unit to 0.