Weapon: Pistol crossbow (Secondary > Special)
Attachments: Poison Bolt (required)
Heist: The Diamond / No Mercy
You may also use the free melee weapon "Stainless Steel Syringe", though this will take longer.
Simply run around shooting cops with the pistol crossbow, picking the arrows back up as you go along. Make sure you kill the cops with the poison and not the actual bolt by doing too much damage, kills like that may not count.