i don't think there's a boss whos the most difficult and is hard. The only thing you need is time and patience, in that way a boss isn't hard to beat. A boss only gets frusterating when you don't have the logics to use the magics right. Well, the most time pressured boss I had was probably Penance. It's one of the most time required bosses of all games I've faced so far(one exception is Gabriel Celeste from Star Ocean 2) but this must be one of the hardest bosses of all time, or at least the most advanced.
"Close your mind,
and
Open your heart."
-Swedish House Mafia