Finished hard difficulty, currently starting Revengeance difficulty, shit's insane, but more or less manageable, I can get past most of the levels without healing items. Ironically the prologue was by far the hardest part, since prologue Raiden doesn't get any of the upgrades you've obtained before, and I'm pretty sure they forgot to take that into account when they raised the damage so high. Literally everything a Gecko does will one-shot prologue Raiden, and there's a Gecko on the very first screen...
But Revengeance mode forces you to utilize all of your tools, I've almost never used ripper mode on my hard play-through, but for Revengeance I always make sure to use ripper mode to eliminate or disarm tougher enemies first, or use sub-weapons to gain the advantage.
Favorite boss fight is
by far, the music, the set piece, the boss design, it was so good they had to devote an entire chapter to it because they know players will want to replay that fight all the time. I never use healing items, sub-weapons or unique weapons against Sam, it's true a samurai duel.