I went back to Musashi to complete the 100 monks lair, and I think I've figured out how regular enemies are scaled in the game. Samurais and ninjas in Musashi died much faster and dealt way less damage than the ones in Kai, so it seems that the enemy scaling is area based (probably tied to the act said area becomes available). The more "advanced" an area is the higher the stats of the same enemy.
This is good news, because it means that leveling up (and therefore xp items) are useful. However, it would have been better if each area had different enemies, or at least used pallette swaps to make things more clear, because the (apparently) same enemies still posing a challenge after 25 levels gives the player the false impression that he/she isn't making any progress.
The system is probably derived from the one in Odin Sphere, where the stats of the enemies of each area were adjusted by the act you were. However, in Odin Sphere it worked much better because you couldn't visit the same area in different acts with the same character, so the "I'm fighting the same guys from the beginning and they're still as hard as always" wasn't an issue.
