Biggest mistake you made
For me, the big dummy move was not farming lethal from dark fae earlier. I have let that rotation pass by a few times without maximizing my silver keys and I really regret it now. My thinking was "I can't do this on auto and win 100%" so it is just too much work. But now I recognize that you don't need a 100% win rate to farm her. Just make a team that CAN beat her and put it on multibattle and walk away. You will have a handful of extra keys after to do it again.