You have to be in Firefight Matchmaking on Heroic difficulty. You have full customization options at your disposal so there's no excuse not to get this. If you don't want to use customization use a Plasma Pistol Overcharge followed by a headshot. Otherwise set all waves to Elites, make yourself invincible, and jump right into the action with your weapon of choice."
Since when have you been able to set invincibility in matchmaking Firefight?? Are you confusing this for custom games?