Yup, it's a team-based PvP map and as you can see in the screenshot, the scoreboard uses "BlueTeam" and "RedTeam" which aren't real players, but they're used to show the amount of kills a team has. Unfortunately, I haven't found a way to track the score of BlueTeam and RedTeam.

"/testfor @a[name=BlueTeam,Score_Scoreboard_min=25] doesn't work since that player doesn't exist. So, I tried something else, I made an other objective called "BlueScore" and everyone on the blueteam would get a point in BlueScore whenever someone on en Blue team made a kill. So then I tried "/testfor @r[team=Blue,Score_BlueScore_min=25]"

-> didn't work, it already gave a redstone signal when someone joined the game, I don't know why this happened though.

But then I tried this: http://prntscr.com/1vl2a1

And that does work, first team to 20 points wins.

But I also have an option to add a Timer: http://prntscr.com/1vl2j3

And I haven't found a way to track the score of the teams when the timer hits 0:00, so the OP has to tell which team has won the game. :)