Most of the bosses seemed fairly easy where I could beat each in two or three tries or even in one.
The hard bosses? I thought Four Kings was fucking ridiculous. You have to be able to deal massive amounts of damage in a short time while also reeling from their massive hits. The only way I managed to do it was by completely disregarding a shield, putting on the heaviest armor in the game, and swinging away while chugging the 20 flasks I had. If you can't kill at least one of the kings before the third one pops up, you're basically done for. I did manage to beat them in three tries but my adrenaline was pumping the entire time. Maybe it was also because the bonfire was so fucking far away.
All-time hardest would have to be Gwynn, Lord of Cinders. He took me a grand total of ten tries. It's a fight where you CANNOT fuck up. You can only hit him once (AND ONLY ONCE) safely when he tries to grab or when you're close enough after he does a kick. He's fast as fuck and even with 190 stamina, his chain attacks would break through my shield and deal 1/3rd of my life bar. If you need to heal, you have to wait for a very slim opening and if you don't hit that opening correctly, he'll smack you really hard. The rocks around the room were a god-send because you could strafe around and he would hit them instead of you but it wasn't guaranteed. Also, I do not see how you could do that fight without the back-flip ring. The regular rolling animation takes way too long and isn't fast enough to effectively dodge his attacks. I ended up dying numerous times on his last sliver of health because i would accidentally roll into a wall and if you can't keep moving away from him, you're fucked.
So, who did you find hardest?