First playthroughs i can get bosses in under 10 attempts a lot of the time because im a completionist and am overleveled. A lot of bosses towards end of game end up being just 2-3 attempts.
When i do SL1 challenges though then that's where the pain comes. I haven't done SL1 elden ring or bloodborne but from DS1-3 the hardest boss for me was nameless king. At around 40 attempts. It just took me a really long time to get the the timing of his one-shot sword strike down π
I did do the DLCs and i don't recall any of them being too bad. It did help in general though i had already done plat so had gone through the game several times and had practiced most bosses and was familiar with their moves before doing SL1. the problem with NK was just that i had never bothered learning the timing of his sword attacks and the one that one-shotted me i kept dodging too soon or too late.
SoC for me was pretty similar to NK but i found it more forgiving since there's more strikes that won't instant kill you. I do think once i started playing more patiently i was able to stop trading hits and that made the difference.
I don't recall trouble with twin princes or champion gundyr. Unfortunately i think i deleted the folder that had all my death tracking for each SL1 run π