Any boss in Deus Ex: Human Revolution.
He interrupts most abilities anyway IIRC, I beat him by just spamming shield and countering. The hardest RPG boss.. Not sure, I don't really play many RPGs. Bowser X from Bowser's Inside Story was pretty bloody hard, though. And the second, purple Lavos, I suppose. (I think it was an optional boss)Captain Hook in BBS.
Why you may ask?
Because I had only water type abilities on me which didn't do shit on him. Then, a little after I beat him, I unlocked ignite. The most frustrating boss I have ever fought.
Well, I've played way too many RPG's, but from the recent years the hardest one was the Star Ocean: Till the End of Time for me, including all those bosses I had to kill on the way (lucky me managed to get all the bosses in the way before the final boss battle... T_T)
Well, on Tales of the Abyss I managed to beat Nebilim in all difficulties since I just love the combat and stuff (and I wanted the outfit you get as reward for beating her), although it took a lot of time and effort to do it. .-."Well, I've played way too many RPG's, but from the recent years the hardest one was the Star Ocean: Till the End of Time for me, including all those bosses I had to kill on the way (lucky me managed to get all the bosses in the way before the final boss battle... T_T)
Do you mean Freya? Given the right strategy she's not massively hard...i'd say that something like Nebilim from Tales of the Abyss would be more challenging, at least i found her to be. (that's having beaten both).