The Arena is found in Pephka, the region in the bottom right of the world map. Defeat all five enemies in the Arena to become Champion and get the "Are You Not Entertained?" achievement. The last enemy is Level 50, but you can defeat him at Level 45 with good gear. At Level 50, the enemy is not too difficult to defeat. Note: Defeating just the Level 50 arena champ is not enough to get the "Are You Not Entertained?" achievement -- you must defeat all five enemies.