As suggested by an anon
Two players from both teams are picked as the Boxer, and are forced to switch to Heavy and go melee-only (ideally forced to use KGB).
All other players are stripped of weapons/go into friendly mode/etc (can't kill anything), and before the two boxers fight, they all go stand in 1 of 2 areas - one red area, and one blue area.
If the RED heavy wins, all of the players standing in the blue area are killed; if the BLU heavy wins, all of the players standing in the red area are killed.
As suggested by an anon
Two players from both teams are picked as the Boxer, and are forced to switch to Heavy and go melee-only (ideally forced to use KGB).
All other players are stripped of weapons/go into friendly mode/etc (can't kill anything), and before the two boxers fight, they all go stand in 1 of 2 areas - one red area, and one blue area.
If the RED heavy wins, all of the players standing in the blue area are killed; if the BLU heavy wins, all of the players standing in the red area are killed.