As far as I know, for part of volume 6, comparing it with volume 1, Alpha attacked with a strong magical attack, which not only defeated Milia's transformation, but also negated part of her extreme regeneration, that is, in the sense that it turned her carbon, returning her to her human state but dead. Then the cult took Milia's deceased body, somehow reviving her as a biological magic weapon, until Shadow came and returned her to normal.
Of course, the manga puts it differently, and for some reason it was placed that way, since all it took for Alpha was a cut in half to finish off the monster/Milin, and return her to Normal. So I would say that the manga version enhanced that ability.