Legendary Panacea IS achievable if... (spoilers ahead)

[Unyaa]

( This is Year 4 )

you didn't remember to do the stuff in what was the Fallen Ruins before it became the fallen ruins. I don't know if it's because I have some DLC or not (or maybe because this is the DX version). I went to the first area of the Palace of Paupers. Use a dowsing rod. I did a relic search. Pick the Rare Medicine

Before, it seemed like everywhere online saying that if you didn't do it pre-Fallen Ruins, you were screwed. I just found it. Dissassemble the Elixir Base when you get back to the Atelier and now, my last assignment is complete