Stuck in the Fourth Chapter

So I finished the first three chapters for all 8 characters but now that I'm in a few of these level 45 areas, I'm absolutely getting my ass kicked. Basic random encounter enemies are just about wiping my party out with some of their moves. I'm getting ready to throw in the towel since this game has already been super grindy up to this point but I thought I would reach out to some experts first. For reference, my main is level 55 while the other party members are all level 43-45.