Dark Souls 2:
The Pursuer: 50+ attempts on my first playthrough, i never managed to pull off the parry trick, nowadays i can beat him rather easily
Fume Knight: 80+ attempts over the course of 3 days
I haven't tried Dark Lurker, Ancient Dragon or King soandso yet (and i don't care to).
Bloodborne:
Ebrietas: 50+ attempts, the long-range magic attack always gave me extreme difficulties
Rom also wasn't exactly a walk in the park, until i watched a couple of videos and used a different weapon
Martyr Logarious: don't remember how many tries, but quite a few. Fell off the roof several times. Enjoyable fight, though.
Jedi Knight - Mysteries of the Sith:
Kyle Katarn: yeah...