I had no problem killing Graven on Hard. I died a couple of times though but not as much as when I fought against Bone Cleaver 2 on Hard.
For Graven, i used the the huge two handed sword only and pressed Y 1-2 times every time while rolling or jumping from his attacks. For the tentacle parts, I simply rotated the gear a little bit each time, otherwise, I ran in a circle at the same direction as Graven. When I reached the gear again, the black oil men were at the other side of the gear so i had plenty of time to rotate the gear.
I just beated Hard mode today, by the way and I already started King mode with all 3 cheats enabled
. I am at the first boss (Bone Cleaver)