I found the battle system was complex, yet still action based. If you mash buttons too much, you'll regret it. But you have a lot of options, you can switch between characters, and the timing is very important.
A lot of the bosses I've faced so far have been pretty tough, and I'm only on normal difficulty. Some of them have specific tricks to defeating them. One has a weak point that's covered in armor. You do hardly any damage to him, until you break the armor and expose the weak point. There are good audio and visual cues that hint that you're supposed to destroy the armor segment, but it's not overly obvious.
I am having fun with the battle system, battles are interesting and after a few hours, you have a wide range of play styles to choose from between each of your party members.