Via Infinito doesn't get hard until level 80, 40 if you have a low level. The only bosses that I've had a bit of difficulty or have been said its hard are:

Black Elemental
Concherer
Chac (its stone stare is so strong it destroys your units and stones even with ribbon)
These 3 also appear as random battles in Via Infinito level 80+

Angra Manyu (more annoying than hard)
Paragon (I wanna fight this oversouled, is there another way to fight it?)
Trema (same as manyu)