How do I make a match timer for a fighting game?

First up, I recommend using a _G.playersAlive = {} variable. This makes it accessible anywhere. In one script you could do this:

_G.playersAlive = {}
local matchTime = 60 -- In seconds
local plrs = game.Players:GetChildren() -- Sets up a table filled with players

for i,v in pairs(plrs) do
	table.insert(_G.PlayersAlive, v.Name) -- Adds in all the players into the table
end

for i=1,matchTime do -- Sets up a loop that will run 60 times (that's the matchTime above).
	if #_G.PlayersAlive == 1 or #_G.PlayersAlive == 0 then -- Checks to see if the # of things in _G.PlayersAlive = 1 or 0
		break -- If _G.PlayersAlive has 0 things in it, it breaks the loop and continues running the next piece of code.
	elseif #_G.PlayersAlive >= 2 then -- If the # of things in _G.PlayersAlive greater than or equal to 2 next line of code is run
		wait(1) -- Waits one second
	end
end
-- Down here you can add code for after the match timer ends

Code to detect if only one player is alive:

game.Players.PlayerAdded:connect(function(plr) -- Fires when the player joins the game
	plr.Character.Humanoid.Died:connect(function() -- Fires when the player dies
		for i,v in pairs(_G.PlayersAlive) do -- Sets up a for index,value loop
			if v == plr.Name then -- Checks to see if the current value in the table is equal to the name of the player.
				table.remove(_G.PlayersAlive, i) -- Removes the player from the table when the player dies
			end
		end
	end)
end)

Hope I helped! :)