After being invited to the Fort by Caesar, the player can speak to Otho at the gate of the arena, which is overlooked by Caesar's tent in the upper area of the Fort. Male couriers can choose to participate in combat. Women are not allowed to fight in the Legion, and female couriers will be told to "know their place."
During combat, only a machete and gladiator armor can be used; all other equipment is removed from the player's inventory. There are three rounds, and after each one Otho must be spoken to again to begin the next.
After each fight, your items are returned to your inventory all at once, but each one has its own on-screen pop-up message. These messages can take several minutes to clear. To avoid this, simply save the game after a fight and reload; this will cancel the pop-ups.
After killing Stella and finishing this quest, most of the Legion soldiers around the camp will start commenting about you saying that "they had never seen someone killing an NCR Ranger in single combat."
Items dropped inside the arena beforehand do not disappear or get removed into the footlocker, making for an easier fight if the player drops any hold-out weapons, or a safety line if player drops chems or consumables, if you find yourself struggling.
Behind the scenes
Laurifer gladiator loosely translates from Latin as "victorious gladiator."
After fighting against the two slaves, you can put your holdout weapons into their inventory. If you immediately start another round in the arena (against Sergeant Andrews) before the slave's bodies disappear, you can get your weapons from the slave's inventory and fight him with your holdout weapons. This can also be done to fight against Ranger Stella. [verified]
As soon as a fight begins, you can quickly run backwards out of the arena before the gate closes. This allows you to grab your items from the footlocker. You have to jump back into the arena because the gate has already closed. This can be used to acquire a weightless armor and machete that cannot be removed from your inventory by anything other than killing the slaves. [verified]