What Shingi suggested was possible, at least in theory. Lucy was still partially an Archangel, but Mephistopheles had corrupted it with his demonic power. Sealing the power of Mephistopheles would lead either to Lucy's death or her Archangel's self to get restored. It depended on how much it had damaged her usual self after all these centuries.
"And you want to tell me you know how to do that? You, a mortal? Even Soren hadn't even attempted as it is impossible to even for her."[Avea]
"I see it is a she now. Well, it was indeed impossible, as everything was until they became possible." [Shingi]
Soren was the God or Goddess of Magic and one of the few Old Gods who kept changing their looks. However, Soren was even changing their gender. That was why some people considered that there was a God of Magic, while others were a Goddess, and a few considered it a couple. Even some fanatics went to the extreme to others that their opinion about gender of Soren was correct.
