Now it worked for me and a friend with this method (we both don't have the swedish spe-ops):
Do the steps written in the first thread with your glitchy class. Means choose that class, which ou needed last to get your achievement normally the first time...for me it was the god damn demolition crap class.
Play to 49xxx points, back out, delete everything, reinstall the update, the hd-conten, then join multiplayer and download the online pass and at last spend tokens (doensn't know if this affects the cheevo). Then go into a lobby and fullfill your 50.000 points.
If you don't hit the 50k in that first joining round, you should earn some tokens with another class and repeat the reinstalling process. This can be extremely annoying because sometimes you join a round with only some seconds left.
Good luck to all!