Battle Brothers

Arenas are located in some of the southern city states with vaguely Arabic-sounding names. You can fight there once per day, but you can't loot afterwards. Usually you will have to choose 3 brothers from your company to fight. A fairly easy solution is to pick the Gladiator origin and to travel from city state to city state in order to get the achievement. I've simply not invested the time in 10 wins yet, which I've not unlocked the achievement myself.