Yeah, Randal the Vandal is totally the right answer here, but I also voted Quodron. Though technically none of the actual bosses are difficult it's the mechanics and/or adds that accompany them that make them so hard. If I had to pick a straight up boss regardless of mechanics around them it'd probably be Aksor in the Winter's Run Strike, since he's actually like a real foe to fight that moves around more, is faster, and has more offensive options than most others.