Very tough, but rewarding
Don't get put off by the grindfest that seems to be the first hour or so of the game. By the time you reach the 6th and 7th levels you will find that simply maxing out attack just doesn't cut it, and you will need to use team synergy if you want to survive. The waves of 1000+ HP devils at the end of the 7th level was particularly challenging to me, and the battles after that appear to be at least as brutal. My main strategy is to have my Kings use Spiritual Healing to massively increase MP regeneration while my Kitsunes blast away with Meteor. However, leveling up characters and unlocking new teammates is quite a lot of fun. Also, the developer was very smart in allowing us to shift around skill tree points at any time during combat. This one seeminly minor gameplay feature gives a huge amount of leeway for experimentation and unique strategies with units.
If you're a fan of fantasy strategy games and want a serious challenge, this is not a game you should pass up.