Disaster "Rags" are probably too hard for you. Disasters are raid battles, and the "Ragnarok" difficulty there is the hardest currently. They've been renamed to Ultimate in the Japanese version by now, though.
But you should farm the raid battles in Standard and Expert, though. Ultimate difficulty only drops the Assault weapons, Standard and Expert drop three kinds and only one of them is Assault.When you are new? Yes. As your account gets more powerful? No.
Skill levels make a big difference, but until you start getting some serious skill-levels in Assaults, it doesn't really matter.