Earn points by defeating other players. All other players on the map are enemies. The player with the most points at the end wins.
It seems that the higher one's ranking at the end of the match the more experience is awarded, with significantly more awarded to the winners winners who are denoted by a star next to their name at the results screen at the end of battle. Furthermore, increased experience is awarded the more total players are in the match.
If there are not enough players, there will only be two winners than the usual three winners. If there are only three total players in the match, there will only be one winner.