i didnt get lightwiehgt 6 until 10th prestige, i just used it on alot of rushing classes in search, barebones, demo, domination. Just dont try and only go for it, try to get challenges for your guns, other perks like sitrep. I got sitrep 6 while getting lightweight, i started at about the same time and by time i had sitrep 6 i had 175 on lightweight.
and i have a question, does running into a wall constantinly count towards lightwieght/marathon?