The boss fights start around 36:05. There's 5 of them in a row at the end of the Chapter.. A lot of running around at the start and I went back to get the Dark Dragon Blade.
After all the sidetracking I managed to make it through all 4 Floors in one go using the Dark Dragon Blade. You'll definitely want to head back to the shop before and after the Imperial Palace. You won't get a chance to restock after this and Vigoorian Emperor is virtually impossible without a decent inventory.
I got killed quite a few times by the Berserkers on the 3rd Floor.. You can also get pulled back through the floors here, and the ghost fish and enemies respawn everytime. The Dark Dragon Blade destroys Berserkers but you get the 50th Scarab AFTER the 3rd Floor. It's basically only good for the 4th floor or running back through any challenges you missed. It's sadly ineffective against the final bosses compared to TDS or Unlabored Flawlessness.