The only realistic way Shadow could win that fight, was if he had the super emeralds, and wasn't fighting like an idiot...but he is a complete combat moron, kinda like Vegeta, but he is alot worse. And the fact Vegeta got back up and walked after the beating from kid Buu, counts as holding his own in my book. Not like after Frieza beat the crap out of him, and he couldn't even move without coughing up blood. And hell, we could throw in current canon, cause Vegeta managed to take a couple hits from Beerus, and even slightly hurt him. Vegeta is a durable little shit.
And I know that omnislash was from different angles, but like I said, that sword he had in AC, housed 7 other swords with it. The Death Battle, he was using the Buster Sword, so he's not going to do that same trick.