For this, each game type has a score you must reach. Performing the objective is the best way to get this. So go for whatever you are supposed to do and you should get there after a while.
Deathmatch style games are probably the hardest way to get it as you then just have to be one of the top players. Whereas anything with an objective means even if you are not getting kills, you can improve your rating.
From what I can tell, the rating never drops. So just keep working away at this and hope it comes by the end of the match. I have had 2 golds and 1 slayer rating and only played for an hour and a half.