Hiruzen wins this low-mid difficulty. Zabuza got out done by part 1 Kakashi who was nothing compared to old Hiruzen which is evident when Kakashi crapped his pants after seeing Orochimaru but Hiruzen stood up to him as well as the previous Hokages.
Hiruzen's scent means that he can locate Zabuza in the mist easily just like how he countered BoD. His earth walls grant him protection from Zabuza's suiton.
I think Emna is more deadly than Zabuza's blade due to the hype surrounding Emna and because it is much more versatile and harder. Zabuza can't hurt Hiruzen if he goes into his cage form. Emna can also extend itself so Hiruzen has the range advantage in CQC.