Note: Adjusting the difficulty of the game makes the entire Arena questline much easier (See “Escape from the Imperial Sewers”).
To join the Arena, simply go the Arena, enter the door to the left, and go down into the Bloodworks. Once you’re down there, look in the room to the left and speak to Owyn, the Blademaster. Tell him that you want to be a combatant and he’ll let you join the Blue Team at the rank of Pit Dog and give you a new suit of armor. You must wear this type of armor for every arena match. The achievement will unlock as soon as the initial conversation with Owyn is complete.