Quest 183

Walkthrough

 * Talk to Jakov. Sainz is mad salty that he got dunked on and wants a rematch.
 * Head to Old Orseco Arena in Orseco Highlands to beat Sainz in a fistfight. Stay equipped, though.
 * You'll be treated to an event and a boss fight. Defeat the boss to learn the true enemy's identity, and also the true identity of Masked Buster.
 * Talk to Jakov to complete the quest and receive your Battlemaster Proof.

The Battlemaster Proof allows you a chance to gain a tension level when you take damage.