there community voted on patreon for endgame difficulty as skulls like in Halo instead of a New Game+ type of end game. So currently there is nothing to do like you want, apart from tweaking your save file. It's in appdata
She's on the last floor of the jungle, she appears as a green skull instead of a normal one. She should also have a star on her iirc. If you already beat here she won't be there
