I wasn't really a fan of challenging games for the longest time. I still played some like DMC3 and Ninja Gaiden but they never felt that difficult to me and I didn't playing the hardest modes
Demons Souls was the first time I just had to confront that and it wasn't so much the boss but getting through 1-1 and the boss
In recent years the Valkyries in GOW, the 3 Berserkers in Ragnarok, Martyr Logius in Bloodborne (randomly one of the few I actually found frustratingly challenging) and Radagon in Elden Ring