Mount & Blade II: Bannerlord

To get this achievement you have to be first in the arena leaderboards. You can chech the leaderboard by going to an arena in any city. I recommend getting this early on because the person at the top gets higher after a while. To find out when the next tournament is go and talk the the tournament master. Before you do this save just so you can reload your save and try again if you fail. If you have enough gold you can max bet to get more winnings. I hope thus helps.

Just some tips since it's pretty straightforward:

The main settlements have tournaments fairly often compared to the last Mount & Blade. An easy way to tell if a settlement is having a tournament is the Helmet Icon next to the settlement name. This usually only appears if you are close to a settlement.

You can also ask the Arena master in any settlement where a tournament is happening. He usually gives you the name of two settlements nearby. Sometimes you won't be able to make it in time depending on the time of day.

While participating in a tournament your companions will sometimes join you. Your companions just like the player character get to use the same armor that is equipped in your inventory. So that means they can be harder or easier to take down when you are pitted up against them.

I personally thought that Battanian tournaments were the easiest and fastest due to the lack of horses.