Covenant-cide - Kill 120 enemies in Firefight Matchmaking - 3200cR
Scoreattack x2 will help you here. Just enough enemies to get this in one run-through. Avoid Installation 04, AI while helpful, are kill stealers.
Be Their Huckleberry - Kill 50 enemies in Firefight Matchmaking with the pistol - 1000cR
I'd say Gruntpocolypse if you're looking to do a couple of games today, but you can get all the FF challenges in one game. Do you SAx2 and pick a loadout with a pistol (I'd suggest shotgun) then, lay waste, after getting a spree or 2.
Points, Points - Score 20,000 points in a Firefight Matchmaking game - 1750cR
Every Medal you get gives you a little boost. If get a headshot rather than a kill, extra couple of points. Same for beatdowns, assassinations and Sticky kills. But, what you want to do, is build up your sprees. Killing, Shotgun & Sticky will be the most likely sprees. These add a multiplier to every kill you get, boosting your overall points.
Killing Spree - Earn a Killing Spree in multiplayer Matchmaking - 2000cR
Living Dead for easy kills, just take them out with the pistol when they're far away, and blow a hole in their chest when they're up close. Of, in regular games, stay away from the action in an objective game, but within range of your gun and within sight of the objective.