If the medalian is anything like the shell that Ursula kept Ariel’s voice trapped in then all you would have to do is smash it. (Only thing I could think to compare it to!) Then the souls would be released, soar back to the body in which they were sucked out of and they would be saved/ revived.
I hadn’t thought about the Little Mermaid but using a really BIG hammer would probably work too. It would be awesome if Thor’s hammer showed up in FT world and Emma used that to destroy the amulet.