[Suzerain of Sheol]

I do have a hard time with Dragonslayer Armor sometimes, which is annoying because he seems like a really easy boss, he only has like five different attacks, but his reach and aggression can definitely catch me off-guard at times.

Also Blackflame Friede in the DLC was another one that gave me an insanely hard time. The first two phases are pretty easy for me, but I'm just so afraid of her in her final form that I can almost never get a hit in on her before she inevitably kills me. So far I've only beaten her with Chaos Vestige spam, which feels a little like cheating.