The Hydra was rather small compared to the boss summons. Orochimaru was about the size of one of it's entire heads. Orochimaru was practically a speck on Manda's head.
In terms of abilities, I found it odd how the Hydra didn't fall into a genjutsu after getting cut several times, so it's abilities are a bit inconsistent.
Assuming it works on them, Katsuyu and Enma probably have the best chance. Katsuyu can split into thousands of parts. Kirin is a one time hit, while an acid spray does damage over time and could potentially wear away at Susanoo.
Enma has a virtually indestructive pole form which should prevent it from being stabbed. However, Enma did worry about the Sword of Kusanagi and this sword seems to be superior. However, if it can't pierce, Enma could very well win.