I just thought about it, shizuku can probably negate ryuko's regen with soul demolition, soul demolition searches for her opponents source of power, which she then cuts it, preventing them from using it.
Pretty much anything that's immaterial in living things she can cut, like her opponents magic power, but lets just say power in this case, can also cut stamina, mind, soul, and considering spirit magic doesn't just allow the user to interfere with the soul, but the immaterial in general, things like energy inside the body of a person such as: heat, electricity, thought, consciousness, and memory, and shizuku is said to be able to cut whatever she wants... Yea ryuko's power gets negated.