I too am struggling with this quest. Some one just gifted me a tip that I will in turn bestow upon you. When you play crucible, play with any gun you like. If your match has gone well and your team is winning, switch over to last word at around 5 minutes left in the match, and hang back in the weeds. Hide if you have to. For some reason, your progress will go up after the match. DO NOT DO THIS IF YOU'RE HAVING A BAD MATCH. This process is slow but it works.