For me, a very casual player, there are certainly some bosses that are much harder for me than others. Having said that, what @Ruff.3027 points out is pretty much my own experience as well. It frustrates me, not only because I know that I'm not that super of a player, but also because it doesn't help me to improve. So, I typically try a boss once and then not really go back to it. For me, it is not so much fun.