Howdy!
Looking deeper into this issue internally we do not have any outstanding known issues with this boss and the client freezing. Your Dxdiag is reporting an AppHangB1 this can be a result of the system running out of resources or a conflict occurring.
I’d recommend trying the following steps just before your next run-in with this boss to see if the issue continues or not:
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Perform a full User Interface reset following the steps Here.
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Try closing background applications to help free up system resources and help avoid third party conflicts following the steps Here. (Restart the PC.)
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If you overclocked your CPU, GPU or Memory try reverting changes back to default.
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Try swapping your game from DirectX12 to 11 or the other way around depending on your current settings. You can do this by logging into your character, press “Esc” → Menu > System > Advanced > Graphics API (Restart the client to take effect.)
Thank you!