After you have defeated a couple of mercenaries a quest will unlock that tasks you with finding the arena. There is a note that comes with the quest to help you find it. The quest is called They Just Want Cruelty, recommended level 25.
The Arena is in Pephka, at Pirate Point. It’s the furthest island to the South East corner of the map.
For this achievement you must battle your way through a number of waves of fighters.
There are five bosses you must defeat to become Champion:
Klaudios, King of the Bandits – Level 22
Belos, the Beast of Sparta – 29
Evanthe, the Huntress – 36
Titos, the Rock of Athens – 43
Vasilis, the King of the Arena – 50
TIPS:
- I would suggest being at least level 49-50 before trying to take on the last two bosses.
- Beware of traps scattered across the area which can be used to damage you enemies - Spartan Kick can result in a nice trap combo.
- Be sure to use your abilities - Shield-Breaker and Second Wind are vital for the final two bosses.
- It is best to have gear that prefers Warrior and Ranged combat styles.
Difficultly doesn't matter, so playing on easy won't void the achievement.
Solution sourced from Apache117X:
https://www.trueachievements.com/gamer/Apache117X