Tobirama beat MS Izuna who was said to be equal to Madara.
by no one whos an actually reliable source. from izuna's mouth, he expressed amazement ANYONE could be on his brother's level. and unlike hashi who is portrayed consistently as slightly above ms madara, we got nothing from izuna. except the vague statement from itachi, whos not a reliable source, did not live in that era, and was deliberately manipulating sasuke to hate and mistrust obidara at the time.
just because choujiro said sasuke killed zabuza doesnt make it true. and just because zetsu said itachis invincible does not make it true. no one else mentioned anything about how izuna was this powerhouse. and the only proof he is comes from the mouth of someone who had every reason to lie