My NL ign: CookieThief always crashes on Ephenia. When Ephenia has 50% hp, horizontal lines appear on her then my client crashes.
There is an explanation of why it crashes. https://royals.ms/forum/threads/ellin-forest-ephenia-crash-bug.194056/ So basically if the fight takes over >10 minutes, it crashes. I suggest partying up with someone so that you can kill it under 10 minutes.
I encountered the horizontal lines issue today, though my client didn't crash and I was able to finish these runs (I have crashed on other runs but this issue didn't happen on those runs).