Sorry for the delay in getting back to you, domerdel. That's a pretty tough boss even at those levels, but what you'll want to do is be sure you have the ribbon on that you can get from the main story's final dungeon (along with the best armor for the rest of your party), haste your healer soon as possible, and just keep everyone healed as much as possible while pouring on the damage.
Might take some RNG luck, but persevere, and you'll get through.