Chapter 6 Leviathan Death Bug

I'm currently playing on impossible mode, and I just killed the leviathan in chapter 6. After I killed it I came back inside and saved my game. When I click my objective button, it still says 'Kill the Leviathan', but the door to get back to it is locked. Since I'm playing on impossible mode, I only have one save file. So I can't reload a previous save file. What do I do?