Many bosses are really hard and have racked up a dozen kills per player.
What boss has the community labeled as a pushover that you've died an embarrassing number of times to?
Crystal Sage from DS3 has killed me at least 8 times every playthrough, but every forum I've visited has made them seem easy.
The first Headless I encountered in Sekiro. I was seeing enough progress and understanding what was being asked of me just enough to be stubborn as hell and kept trying to fight it head on, without having any knowledge yet of any helpful items that make the fight less rage-enducing.
Outside of Fromsoft, my NG+ encounter with the green swamp monster thing in Lies of P seemed SO much harder than the first time. So I'm not sure if this boss is considered easy or hard, but I didn't consider it to be very hard going in for a second time and got quite stuck for a while...