Hmm, how interesting that I reached the boss battle with the same party as you! In my case, I found the fight to be quite manageable even in hard mode. I believe you are suffering from what one might call, a "skill issue".
My suggestion is to attempt another run with characters that are more suited to your playing style! I'm sure with your almost accomplishment of beating the game in hard mode prior it is very possible for you to win under different circumstances.