This game is awesome! The battle system is really fun, and I think overall it is fair but challenging. Though there are some things I have to say
1.Maybe add an attack that turns you around, maybe a back kick. A lot of times I wish I could turn during my attacks.
2. The first boss can be defeated without damage easily with a swapping character. Just wait for the dash attack and swap. Maybe give him a ranged attack so you can't just keep hopping out of range?
To be honest most of, if not all the bosses can be done the same way once you have an idea of their pattern such as first boss usually follows pattern of Sword, Spin, Dash. or the second boss, who is in my option the hardest boss since he both summons and has the shield which is a pain to deal with unless you have multiple attacks queued or poison. If not a swap position attack a rotation attack can be just as effective for them and having a knock back such as Dragon Fist for when the boss is right up on you and you want to do damage and avoid an attack like swing or sword. The third and final boss can be tricky since they bring the ranged element and as well as having some attacks with "Effects" on them such as the freeze arrows on the final or the double swing from the third. Most of the time the thing which will get you is not the bosses but the main enemies from the stages prior to them.