Gonna write up some more lore bits while waiting.

While the Witch is essentially a normal human with immortality, that immortality allows her to do some things normal people cannot, most important of which is free use of "hysterical strength." Our bodies our way stronger than we think, but that strength is self-destructive, so our bodies possess a limiter to keep us from tearing ourselves apart. "Hysterical strength" overrides that limiter in certain emergencies. The Witch has trained herself to override that limiter at will, and is able to heal injuries incurred in the process.

There is a caveat to this healing ability, however: healing is painful. The faster the healing, the greater the pain. Regenerating a lost limb in a few seconds incurs excruciating, traumatizing pain, while doing so over a few hours incurs a dull ache. The amount of strength she uses is often enough to crack her bones or strain some muscles, but the pain from healing it is often bearable, especially if she takes painkillers (which she tries to avoid). This is how she manages to hold her edge against her mortal opponents. Anyone with good healing powers could probably replicate this ability, but unlike a mage, the Witch will never run out of mana doing it. She might run out of pain tolerance though, so she likes to reserve that level of strength for truly worthy opponents.