Like a Dragon: Ishin!

To unlock this achievement, you need to win battles in series conducted at the Arena venue, located at the second floor of Mukurogai. If you're getting there from the early portions of the game (via travel through the Boatman located in southeast portion of Shinmachi in Fushimi), take a left upon arriving on Mukurogai on first floor, and you'll see a set of stairs on your right. Go right, up the stairs, then turn left to another set of stairs that'll lead you to the second floor as well as the doors (the ones with a trio of beggars sitting to the right) to the Arena venue.

From there, talk to the receptionist and select 'Compete' and then 'Knockout Match' which will bring you a series of one-on-one matches against five opponents you need to beat in order to get the whole of the rewards offered. Each opponent you defeat will count to the 10 needed to unlock the achievement, you don't need to beat all five for all victories to count. Be warned though that your lifebar doesn't replenish after every win unlike previous Yakuza/LAD arena tournament bouts, and even opponents at the easiest tier (Baptism By Fire) have some decent weaponry as far as damage goes, so just keep in mind if you want to go at it early in the game.