A Clash of Swordsman.

The Varia are the Vongola's Elite Assassination Squad. They're all odd balls but they get along well and are able to complete any task handed to them with the utmost loyalty to their boss, Xanxus. That being said they're the opponents against Tsuna and his gang and the other candidates for the Tenth and his Guardians. Anyway, Xanxus also has a swordsman, Squalo who fights against Yamamoto in the Rain Battle.

The Vongola Rings are split into two halves and each side gets a half. Then the two people with the corresponding halves must battle in order to get the whole. Which is slightly pointless because they have to do it again in the Sky Battle. Yamamoto has to fight the "Sword Emperor" Superbia Squalo. Squalo has defeated every sword style in the world and his own in built upon those he defeated. As such he can't be taken by surprise because he's already seen Yamamoto's moves right? WRONG! Just when it's looking rough for Yamamoto he realizes that Squalo fought a sword master with another branch of the Shigure Souen Style. This allows Yamamoto to win because Squalo is shocked by his inability to predict Yamamoto's moves. Really, it was Yamamoto's faith in his father and in the style that helped him defeat the over confident shark
 
Afterwards Yamamoto tries to save Squalo from the shark that the Cerevello decided to release into their battleground. Squalo however fights Yamamoto and falls into the water in order to force Yamamoto to escape on his own. Squalo barely makes it but lives due to Dino's care. In the Ten Years Later Arc Squalo seems to be fond of Yamamoto and considers him a sparring partner or a rival because he lends Yamamoto his tapes of all his battles on his way to become The Sword Emperor. These tapes offer Yamamoto advice, help him increase his skills, and put him down with insults all at the same time.