1) New Game Plus is required. Cannot be accessed on a first run.

2) The Empress social link must be completed. Margaret's request need to be completed in the run you wish to fight her in. (If you have all 10 requests saved in the compendium, this should be pretty simple).

3) Every optional boss in each dungeon must be defeated. (Every time you clear a dungeon a new shadow appears at the top. You need to defeat every single one).

4) You need to be going for the True Ending by March 30.
Link to another guide: https://sp.zhabite.com/sharedfiles/filedetails/?id=2130350101

5) When Igor gifts the Orb of Sight item, go back into the Velvet Room. Margaret should give you a secret request.

6) Climb the Heaven dungeon again. Once you reach the top, you can fight the hardest challenge in the game.

Good Luck....