Sometimes you know there is no winning. The difficult conversation needs to happen. It will be uncomfortable for all parties involved. So what do you do about it?
What you do is use that knowledge and prepare yourself to respond well to the coming firestorm. You must let the fire burn and the insults to be slung.
The other party will calm down eventually and your more humane response to the difficult conversation is just more kinds, but you will also stand in a more influential position as the relationship moves forward.
The key is to withstand the initial firestorm no matter how insulting or scary it seems at the moment. Have courage! The amateur fades, the professional stands firm and sees it through.