Self explanatory really: Score 50 kills in any type of ranked match. This will be very difficult, and very time consuming, as so few people play this game online.
If you are prepared to boost, read on. If you are not, please do not read on. I'm not forcing you, and I'm giving you ample warning!
Are you sure you want to keep reading?
Right, you need one other person. Set up a ranked Team Dogfight, NOT a free-for-all. In Team Dogfight, when you kill a target on their side of the map, they will respawn on their side of the map. After a few kills, you will be able to work out exactly where they will spawn, and you should be able to get 50 kills in ten minutes.
Do not choose FFA, as your opponent will spawn on the completely opposite side of the map every time you kill them. You will be lucky to get 20 kills in a game.
Good luck!