What stops itachi from one shotting hashirama with tsukuyomi or totsuka blade. Hashirama isn't known for his speed so he wouldnt be able to dodge much from amaterasu or totsuka. He also would have to always be starting at itachi's feet if he wishes to prolong the fight. Itachi is very quick and a battle genius. If Hashirama decides to go sage mode and whip out the giant buddah, itachi can whip out susano'o and seal it with totsuka