Most bullshit battle in the game. Defeating Isaac is already a chore, but nooooo, Death HAD to have moves edging on instakill.
It's not even that; it's the literal one-two of a grueling battle against Isaac in which most players will drain many of their healing items and take major hits to their Innocent Devils. THEN, with no cooldown or chance to heal and recover, Death shows his genre-savviness and attacks Hector once Isaac has softened him up.
It's totally realistically unfair and a fantastically villainously dick move by Death, and makes his battle the single hardest in the game.
Dracula WOULD qualify, as it is a fairly challenging version of the Count (far more than usual), save for the fact that unlike Death, you can stop, save, heal, replenish your supplies, and grind prior to ascending that final staircase.
Death attacks you only once Isaac has you on your last legs, and Isaac's battle is specifically constructed to drain you and force you to run on fumes.
It's when Castlevania almost became Dark Souls-level Guide-Dang-It hard again. Isaac alone comes closer to being a stereotypical SNK boss than any other boss in the 3D games because of how agile and spammy he is.
I can't even imagine doing that on Crazy mode legit.
I did Action Replay my way to victory in Crazy Mode back in the day, and
even with invincibility, unlimited hearts and the best gear in the game, Isaac-Death is an exercise in sheer frustration and pure rage.