Master is right, if you have more than 2 people in an arena they will team up, even if they are total strangers they will eventually team up against the weakest or against the strongest. Here is an idea of how it could work if you just give it more time:
There are 2 people in each arena ( that means 2 people in the 25 pec, 2 people in the 50 pec . . .).
There would be 1 minute rounds.
Assuming I'm in the 1 ped arena and i kill my opponent before the 1 minute is up i will be promoted to the 2 ped arena after the 1 minute is over.
Assuming I'm in the 1 ped arena and i get killed by my opponent before the 1 minute is up i will be demoted to the 50 pec arena after the 1 minute is over.
Assuming I'm in the 1 ped arena and we don't kill each other by the end of the minute we will both be sent to the proving grounds.
players in the 10 ped arena don't get promoted after killing and players in the 25 pec arena get demoted to the proving ground.
after every 1 minute round, the player with the top score in the proving ground in the last 15 minutes will be promoted to the 25 pec arena.
Only kills in the 10 ped arena will count towards the event. If a tie occures scores in the 5 ped arena will be taken into acount . . .
The event doesn't need to be 3, 8 hour events. It could take place all day and night for 4 or 5 days.
this is just an idea though