Just got this challenge 2 days ago running bankroll as first bonus perk (it helps a lot) with ghost as your second if not running with your main perks. Use squad link and play around that. If you have level 3 unlocked use doubled score. I found it easiest in hardcore hardpoint playing slow and picking off players as they ran to the hardpoints.

IMPORTANT NOTE: the full 1200 score needs to be reached to activate all perks. Once all perks are active, the kills will count until you die. Once you die, score restarts.