Radagon and Elden Beast at rune level 1. The area of effect attacks and the tracking projectiles are brutal. I was able to get a strategy that clobbered Radagon in <1 minute without taking any hits but it required really intense concentration and timing. Not being able to practice just the Beast was the worst part, I hated having to slog through Radagon every single time to attempt it.

Also Sister Friede at level 1 is a strong contender. I don’t remember which took me longer to complete.

I found discussions of this leak from August 2024. This is just bringing attention to it again.