The Mushin 8.2 fight is pretty straight forward and is fairly easy if you know your class. The only really dangerous thing you have to worry about is when he goes to the center of the room and emits waves of fire. This does a lot of damage other than that pretty easy.
The last form of Mushin can be a little tricky cause he can one shot you if you aren’t careful. He also has a lot of health so staying focused is priority here. You wanna keep him snared as long as possible and stunned. Make sure you never over lap your ccs on him.