Changes in Resolution, Windowed vs. Fullscreen, DX 11 vs 12, and so on all did not help me.
What did help was reducing the refresh rate of my Display to 24 Hz and activating V-Sync.
If you crash in that fight too, please test and reply to confirm or deny my theory.

There is some kind of bug that crashes that boss to the desktop.

I fixed this doing the following:

Set control_DX12.exe process to high priority in task manger.

Load game > alt tab to desktop > run task manager > click details tab > find control_DX12.exe > right click the control_DX12.exe > go to priority and click high

Set control_DX12.exe process to CPU0 to CPU3 affinity.

While in task manager > right click the control_DX12.exe > click set affinity > untick all CPU cores apart from CPU0, CPU1, CPU2 and CPU3 and click OK.

Alt tab back into the game and try it.

To reset the settings close the game and when you re-open the game the settings will have changed back.

Thank you very much ! I was getting desperate, as all the fixes failed for me

See answer here: https://www.gog.com/forum/control/ctd_in_anchor_fight

The only thing I had to do was mute the sound. That's it and it worked.

Set affinity to first four cores as you did but game still crashed running in DX12. I used DX11 with first four cores and set priority to realtime which worked like a charm.

So I've seen all your post and i have tried ALL of them. Tried DX11 and DX12 with all the suggestions. I even tried turning the volume down. I can play part of the fight but it crashes about half way though every time. Not sure what i'm doing wrong but it's really frustrating. Can anyone give me any other tips cus these definitely helped me at least start the fight. just can't finish it. Thanks in advance!