There are a plethora class balance issues, but a lot of them could be momentarily fixed by changing boss integers.
A band aid fix would be too lower the boss hp significantly and up their spell casting speed(reduce the time in between their spells)/effectively raise their dps.
Right now bosses are just giant sponges. They are boring to fight and some builds are even invalidated by their solo boss speed. For instance, tank builds are completely useless, at least for the first 60 levels.