Orochimaru stomps mid to high difficulty. I can see it being a pretty even fight until Orochimaru pulls out the eight headed hydra that scales at the minimum to the sage creatures, so unless Kakazu has a totsuka blade hidden up one of his assholes then its gg.
And I hope no one really thinks Kakazu is capable of launching multiple attacks on the scale of Deidara's C0 attack which was just barely strong enough to kill Manda as Manda was still clinging to life after the blast.
Orochimaru would also be able to seal Kakazus hearts with ease as well as tank Kakazus most devastating attacks, as evident by his durability feats in his battle against 4-tails Naruto, who I think also scales above Kakazu.