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]