Just separate 3 or 4 similar GF "arenas" (in one level, ofc) and have one person from both teams battle in each. Then move on to the semi-finals and later, the final, if necessary (if all competitors aren't from the same team after any stage).
Then redo the round or do a new one, giving the winning team a point.

I might as well make an example level and write an article about this when I have the time.