I'm finding Hard + Classic to be a really good mix of fun and challenge. While Hard difficulty really isn't all that tough on its own, the real challenge is keeping absolutely everyone alive, particularly the squishy healers. I've had to even leave out the healers altogether on one of the fights because they kept getting picked off by enemies with a huge movement range.
Also, does anyone think that Frederick makes the game a little too easy? I paired him with Sumia so he gets enough speed to attack almost every enemy he encounters twice and he kills even boss units in a single turn with no troubles, taking barely any damage from enemy attacks. Its a viable strategy to just leave all your units at spawn and send Frederick into the middle of the enemies, where he decimates them all with counterattacks, barely losing any of his own hp.