hmm, i would rank them like this:
S: Egypt, the combination of the lava platforming and the constant enemies is really hard. I fell in the lava so many times on my first playthrough its unbelievable.
A: Rome, The only hard thing about this boss is the constant barrage of mines and the long trip to the bolts.
C: Helvetia, You need to stand near the Formations (that always have their spear out) to make them get off the pressure plates. Pretty easy and you can easily get the hang of it
Greece, Like the Normandy machine but with a bit harder platforming and a long way to the arena. The path to the arena is really easy, you might die once because you dont expect the lava pits and the enemies die because of the machine.
E: Normandy, literally does nothing until second phase. Shoots 6 mines on its last phase and the way to the cable car is ridiculously easy.