You need to get 500,000 proficiency points with the Tonfa (by using it within missions). You can check how many you have by looking at the Dojo mission and it will say how many you have.
Once you have 500,000, complete the mission, just beat the Tonfa user 1 on 1.
Once complete, you have unlocked the Mystic Art, but you need to purchase it on the Skill Tree for the Tonfa, it is on the right hand side!