Hardest to easiest (granted easiest is a relative term, because these are all challenging endgame or bonus bosses):
1. Darkeater Midir: I have never beaten him solo. I know the strategy is to "just hit him in the head, bro," but I can never get myself in position to land any hits before he either dashes or flies across the arena or does a stupid gigantic slam that takes off 80% of my health or outright kills me. I've resigned myself to never killing him solo and just summon for him.
2. Malenia: Like Midir, I haven't beaten her solo yet, but I imagine I could, because, other than Waterfowl Dance, she's fairly manageable. I feel like her other moves are well telegraphed, she has fairly low poise leading to easy stuns, and her health pool feels totally fair. However, when I got to her during my first and so far only play through of Elden Ring, I fought her about 12 or 15 times, with and without summons, and after that, I was done, so I used my mimic tear. But I feel like I could beat her with enough practice if I ever wanted to spend more time learning Waterfowl Dance.
3. Nameless King: He's such a pain in the ass. I always come out with zero estus remaining. He's also an interesting inverse from the above Midir. Whereas I must summon for Midir, I must solo Nameless because the health boost Nameless gets during summons is always far greater than the minuscule damage my summons do before they get killed.
4. Sister Friede and Father Ariandel: Tough and long due to the three stage fight, but I feel like From put in enough weaknesses to make them fair. Overall, each stage has a manageable health pool. Phase 1, Friede is easily stunlocked due to low poise. Phase 2, Friede and Ariandel share a health pool allowing you to target whichever you prefer. Phase 3, I felt like you could dodge many of Friede's dashes and drives to get in some back stabs or at least some easy hits.
5. Orphan of Kos: He's aggressive, but what boss isn't in Bloodborne? I always felt like I had the tools to beat him, but just needed help understanding when he could be gun parried. Once I knew that, I could manage.
6. Sword Saint Isshin: I don't know. I didn't find him as difficult as many made him out to be. By the end of Sekiro, my deflect and Mikiri counter game was strong, so phase 1 against Genichiro went pretty smooth having beat Genichiro twice already, phase 2 was a basic dual except for the inclusion of a gun which you can deflect, and phase 4 allowed for crazy Lightning Reversal abuse. Phase 3 was the only real challenge for me. I had to relearn how to fight him due to his increased range from his spear.
