What is a damage "step," Higure? Is that from C to B or C to C+?
Okay...well, I've rehauled the damage mechanics a couple of times since starting the quest. As of the last iteration (hopefully the last...keep finding weird situations where things don't make sense -_-), it works pretty much like this:
Everything has various levels of "Damage." Homura's Body, for example...
Trivial (Scratches and bruising. No penalties.)
Minimal (Deep cuts or small holes. Minor penalties to physical actions, due to low-level muscle damage.)
Moderate (Wide gashes or gaping wounds. Major penalties often apply.)
Severe (Limbs are missing. Some actions are conventionally impossible.)
Catastrophic (Head and/or whole body is destroyed. Body-dependent actions are impossible.)
Her Soul Gem, though...
Severe (Cracked. Major problems.)
Catastrophic (Destroyed/Broken. She's dead, Jim.)
A Damage boosting Enchantment simply means that, IF you manage to harm them, the degree of harm will go up by 1 stage. The number of stages varies between targets, though.
Edit: More Details -
Thematically, a bullet that would normally create a clean hole would instead create a crater. A blade that would normally cut deeply would instead cut fully through. That kind of thing.
Edit: And Still More -
Repeated instances of a given level of damage being caused can result in those instances being converted into a single instance of a higher damage level.