Spiderdemon in the original Doom.
It's the final boss, but I have never had even an ounce of trouble with it! To be fair, I don't think I've ever fought it without the plasma gun, so that probably does take a bit of the heat off.
Being in complete shock when I got home and discovered that my apartment had disappeared! Of course, that was before I realised that I got off the lift/elevator at the wrong floor.
I have for the majority of my life. Fortunately, I've been on medication for it since 2022 and it has helped a fair bit. I can actually achieve the recommended eight hours that alluded me for so long!!