For the benefit of everyone looking or posting in this thread, THQ has closed the Ranked servers for this game (~Aug, 2011), so the following achievements are unobtainable:
* Top Scorer
However, you are still able to use Player/non-Ranked & Private matches to get the other four multiplayer achievements:
* Killing Spree
* Fashion Police
And on a side note, I was able to get "Noob" in a player match where I was only the 4th player in a 12 player game that was still in its pre-game/warm-up stage. Not sure if that would also work with the other achievements.
Good luck to those of you going for as close to 100% on this game as you are able!