Just play hardcore for every gun but assault rifles. I originally wasn't even going for it, just trying to hit master prestige, but since I like to use different guns so I don't get bored, I found by the time I hit master prestige I already had about 21 guns in gold, so I thought I may aswell get darkmatter which took about 3-4 days to get. I got dark matter about 3 week ago, it really wasn't even that hard to be honest.
