Can't say I know a whole lot from either Soul Calibur or Onimusha. But from what I remember this would be a good match. Use their strongest incarnations I guess.
Yes... Siegfried is more than capable of decimating fortified structures and hordes of warriors if the crystalisation techniques we saw in SC4 was anything to go by... Then there's the Rough Divide type of move in his Critical Edge finisher... I need feats for Samo to compare, since I know little about him though.
This wouldn't be a stomp either way, as little as I know about them, I knew that at least. This should be a damn good fight. Either would fit in with the threat levels of the other's universe.
And none of that was as impressive as the feats a bodiless Nightmare without Soul Edge was capable of, and the explosions were not as impressive as the one he caused.
When said Nightmare fought Siegfried, their blades clashed and it destroyed the entire fortress they were inside, this clash eviscerated Sieg and sent Nightmare into another dimension. It is important to note Sieg has healing abilities now, Soul Calibur then bonded with him and did not allow him to die, healing his wounds instantly. Plus, you know, he can freeze a tower over in a matter of seconds.