As far as I can tell... there are about 5 enemy dragons you have to vanquish. (Or is it 6?) In addition to that, there are five corrupt kings who serve as half-way bosses to the dragons.
The game is broken up into worlds of about 4-5 stages for each dragon. Each stage takes between 8-15 minutes to finish, or at least they have so far.
With respect to character design, the dragons are each a radically different archetype. One is more of a chinese water dragon, one is more of a giant evil snapping turtle, one is more of a serpent, etc. The corrupted kings themselves are actually also interesting pieces of character design as each king has turned -half monster in different ways, from just an arm, to their heads/faces, etc. The enemies are for the most part predicatble fodder: lizardmen, pigmen, minotaurs, spiders, bats and variants thereof.
I just beat the third dragon, and he took me about 45 minutes to learn and to beat. The trick to it is that each dragon tends to have multiple stages to go through (well, 3 stages really), and each stage has its own attack patterns and timing to learn.This means when playing a boss stage, you're likely to go through it multiple times just to learn the patterns and timings of the attacks, and to develop your own strategies to avoid them or turn them to your advantage. It's very, very important to learn the timing of some boss attacks perfectly because if you block right before an attack in Dragon's Blade, not only do you not take damage but the game even rewards you with a little life. This is important when trying to keep your health up through all three of a boss' phases, instead of having your life whittled down slowly.
In that respect, I'm actually very staisfied with these bosses because they are very traditional, and hardcore in the way bosses in NES and SNES games were. I'm dying a lot in this game due to the dragons and kings, but luckily the game's checkpoint system tajkes the sting out while I learn their attack patterns and dragon's get their own boss stage so after you die, you can just restart the stage with no filler to dig through.