One Spartan Army - Kill 400 enemies in Firefight Matchmaking - 2000cR
A few games of anything will work. A few other Firefight challenges so, all is good.
Blast Radius - Kill 75 enemies in Firefight Matchmaking with grenades - 2500cR
Throw them into groups and keep yourself stocked. Grunts and Elites drop a bunch.
This is My Rifle... - Earn 40 kills with precision weapons in multiplayer Matchmaking - 2600cR
SWAT & Snipers for the easy build. Just pick up a DMR, Sniper, Needler or Focus Rifle.
Survivor - Earn 9 Sprees in a Firefight Matchmaking game - 3000cR
10, 20, 30, 40, 50 & 100 for killing sprees 5, 10 & 15 for Sniper, Shotgun, Sticky, Splatter, wheelman and other special sprees.