This isn't as hard as it might sound.
All you have to do is win 50 games in Multiplayer.
I would recommend playing Chaos TDM if you have a hard time in Control or other objective-based gamemodes.
Another tip could be playing Free for all, so the skill of your teammates doesn't effect your wins - also, a 1st, 2nd and 3rd place in free for all, counts as a win.
You'll probably unlock this achievement by just playing the game normally, without trying to hard for it.