Gotta go with Kakuzu on this one. In his split form, Kamui wouldn't be able to take out all of the bodies at once. And if Kakashi tries to Kamui the main body, remember, Kamui isn't instant. Kakuzu has 4 other bodies fully capable of attacking Kakashi while he aims for the blow. Kakashi won't have the time to just sit idly by.
Considering that Kakuzu can also pretty much use any combination of attacks through his hearts, Kakashi will have a hard time avoiding all of them since they're all aimed at him.
Kakuzu takes it mid/high-difficulty.