As far as I know, you can be final champion in the arena after all the other contenders have been champions. First 5 days is Kokeeno, then next 5 days is Magnum, then Toro for 5 days, then Gort for 5 days and finally Elsa for 5 days. After Elsa, on 26th day go to Blue Parrot Inn at 17:00 and look at the betting board and you will have the option to be final champion. You will automatically get challenges and you'll fight every day until Day 30 with others except the day when supposed to fight with Magnum if he has been killed, even if he is shown as a challenger.
You can fight Magnum every time if you choose not to start the Rites of Rulership before arena season ends, or you can prolong the Rite of Freedom by freeing all 5 villages (to prevent others from winning) and not showing guards sigils until you complete fights. Kokeeno will be killed during the first rite and won't fight as a challenger, but he won't join later fights in the arena when Gort enters even if he stays alive (by you not starting Rites sooner).
