It's not 200 super kills, 100 melee kills and 150 grenade kills. Those are the amount of "points" you need. It's roughly 30 to 40 super kills, 20 melee kills and 30 grenade kills. When someone else gets a kill of one of these types on your team it gives around 1 or 2 points, so if you aren't actively trying to get the step done, it will take a long time.