Tough one. I really want to say something from Dark souls but I would be lying. I think the hardest boss Ive fought in recent memory was when I tried soloing the Tyrant zombie in Raiderz a few months ago. You have to have perfect dodges nearly the whole fight or he can very easily kill you. It took me around 10 tries before I finally did it and even fighting Gwyn while completely naked and with default fists doesnt require the timing that fight does