This can only be done in two player mode. Have each player jump on top of a building of some type and start fighting. If one player is knocked off, the other player is the winner. The best levels for this to be done are "Airport", "Waterfront", "Death Trap", and "Space Dock". You cannot do this in "Green Hills" because there is not any place to fight. For existing matches, fight in "Ruins" and battle on top of the little heads.