Really good job on the art style! I'm a sucker for good character design, and I really like what you did with the heros and the raid boss character designs. I also appreciate that you did your own music for the jam, so good job on the atmosphere building!
Not to overanalyze a game jam project (as I, unfortunately, am apt to do), but I think that the initial concept offers some unique, and quite difficult challenges from a gameplay and balancing perspective. In a general a boss in a bossfight has the advantage of health pool and damage output, but the players have the advantage of action economy. In my opinion, it's more fun to play with the action economy because it lends naturally to more interesting gameplay choices - you have more turns per turn so to speak. So, inverting this dynamic and playing as the person who traditionally has very limited choices can lead to uninteresting gameplay if not balanced properly on that knifes' edge.
As a potential solution to this problem, I suggest you break away from traditional RPG mechanics to create unique ways to combine your own abilities together. I'm not entirely sure how this should best be designed, but the end goal should be that one turn for the boss should involve numerous interesting decisions for the player to make. One idea I had would be the ability to set traps for the adventurers where if they make specific actions you can counter them (kind of like trap cards in Yugioh) but there are a billion other ways to achieve this goal.
Good work, and good luck!