Zaraki. Beat 3 stern Ritter easily including 1 who emulated his own power. Also presumably Shinji survived her explosion. Zaraki would do so with less troubles if necessary.
It depends on the extent of prep. If the doctor went in the past he could win, but that's not really prep if they are in a fight. He wouldn't be on the battlefield at the time the battle started so I think that's cheating. However anything short of that and Hao could effortlessly kill him.