Heaven on High Bug

[Kuukaku]

Whenever I'm doing floors 21 - 29, I'm doing just perfectly fine. No lag. No graphic issues. Game is running as its intended. But when I hit floor 30, encountering a Hiruko Boss. The game becomes extremely lagging, the frames are choppy, and to top it off, The actual floor is missing, The walls are gone, I see clear blue skies all around us. And It was really hard for me to fight, it even was even harder for the healer to keep me alive during the whole fight.

Is there a way to fix this issue? I can run the game just perfect fine ANYWHERE else. But that boss.