Online is dead, so you will need a boosting partner!
Host a ranked match on Either Team Base Capture or Hunter/Hunted. I think it will be faster on Team Base Capture, but all kills of the fox will be counted towards this achivement and DOES NOT have to be all in one game.
Here, both players will be The Fox (if playing 1v1), so any kill will count toward the Achievement. A Small map that your fimiliar with will work perfect like Prison Cell or Warzone.
Killl the Fox 25 times and achievment will unlock mid-game when you reach your goal