So i beat Khazan on Expert with the platinum trophy, and clocked at 46 hours. I've seen dozens of boss rankings and my experience was pretty different than most so i just thought id share my thoughts. This list is in order of easiest to hardest for me during my time playing.
16 Aratra (i 1 shot this dude i dont even know if he has mechanics)
15 Blade Phantom
14 Shactuka
13 Yetuga (Wiped several times. Idk)
12 Bellerian
11 Trokka (Very easy to get behind and basically stunlock)
10 Ilyna
9 Reese
8 Elamein (her moves are easy to read but she puts alot of pressure on you and the shield is annoying)
7 Ozma (Disappointed in the difficulty after reading many comments about this fight. He fell on my 5th attempt. Very cool boss and sick moves, but not hard.)
6 Hismar (almost beat on my 1st attempt but then it took another 5-7 to learn the timing of his dives. Sick fight though)
5 Skalpel (Wiped nonstop until i accidently knocked him off his coffin. Gamechanger after that discovery. Cleared on the next pull.)
4 Maluca (2nd phase is brutal. Once i learned to be up his ass the entire phase he melted. He hates pressue)
3 Volbaino (hoe)
2 Rangkus (hoe)
1 Viper (Hoe)