Oceiros - The Consumed King - Dark Souls 3 Boss
I manage to beat Oceiros, the Consumed King on my first try without dying. He was... an interesting Boss. Half-way through the fight you may notice that I remembered how to Block attacks, which drastically improved my sustainability. The difficulty of this fight really falls in your management of your stamina and knowing when he is open for attack. His attacks are predictable enough that you can generally get by without taking too much damage, and I found that when I was hit, it wasn't too painful as he hardly has any combos which can stun-lock you.