Notorious Edit September 20th 2015: According to
Dark2099:
There are no more crucible specific marks, and this now simply requires hitting rank 1 with the crucible reputation.
Once you hit level 18 you get points towards you Crucible score. These points you can use to get legendary gear in the city.
The maximum you can get in a week is 100 so once you hit that you obtain this achievement and cannot obtain Crucible points until its a new week.
When you win a match you get 3 points, 2 points when you lose a match.
If it doesn't unlock, try logging in then out.
The Cap will reset every Tuesday 2:00 AM US PST
Note: If you do not make it to 100 you have to start from 0 again and get to 100..