I've said it a million times on other forums; every guild will have trouble in their own ways. Every boss will be different to each group's experience because of their strengths and weaknesses, along with composition. The hardest part about doing a heroic boss is how different it can be from normal mode, giving players more to do, thus becoming more chaotic. Spirit Kings is a great example of that, and has proven to be the hardest boss in MV. It was a good road block for good guilds before the nerf as well. Will of the Emperor is also very different from normal mode, which gives the average guild a lot of trouble. To really good guilds the fight is still easy, and it only takes a few attempts for some of them, but others just can't handle it. Sometimes a guild will hold off on even attempting some bosses because of their nature or stature, like being the last boss in the instance (or one of the later ones), giving it the illusion that it is difficult. Usually it's one of the earlier bosses that gives guilds trouble, such as Stone Guards or Garalon. In my opinion, the measure of how difficult a boss is would be how many mechanics it has, and how well you have to work with your group. When it's a fight where it's just pure healing and DPS, the fight becomes easier with gear and isn't much to deal with.