Up until recently, I thought all bosses were like those easy peasy civilization level 1-2 giant minibosses and regular microbosses. Menacing, fast, powerful, but in the end, way too easy to take down. Then I had a sort of different experience.
First of all, I challenged a few Tower bosses. The first few were gigantic skeletons, not too hard. Then I got to some sort of giant amoeba with ridiculous amounts of HP that just camped in a corner I couldn't reach (becoming a bat and flying up to it wasn't effective because of the bat attacking nerf, and I couldn't get my Energy Pulse exactly where it was since I couldn't really see it that high up). By the time I thought to build platforms up to it I was already dead.
Then I got to Civ Level 3 and saw a hidden dragon boss on the map. Sure enough, I crossed 4 monotonous snowfields and there it was. Only problem... it was in a small hidey-hole under the surface of the area, filled with a bunch of other regular enemies. I quickly learned what was annoying about the dragons was that they would churn out ludicrous amounts of dragon-fire, which basically filled up the hidey-hole, turning any attempt to engage it into a Touhou game with a hitbox that's a little too large. I learned that the best way to fight that was to stay at the entrance of the pit and shoot, but again, by the time I figured that out completely, I was dead.
This game is nuts. I'm actually playing on regular difficulty, too. Can't imagine playing on hardest.