@Teran: Wow, excellent stuff. The only caveat I'd add is gruntpocalypse doesn't seem to have 120 enemies every single time. I completed a round on this challenge day (forget which map) and I had 111 or 112 kills. Or maybe 8-9 grunts killed themselves via poorly thrown grenades, etc and that's why I had less than 120?
In other news, today's challenges are very tough IMO.
I thought a few of the other days so far this week had easy challenges, but today is super easy. If you still need the weekly challenge, today is likely the low hanging fruit of the week.
750cR - More than a Handful Kill 77 enemies in multiplayer Matchmaking. Just play 4+ matches, depending on how many kills you can average per match. I suppose if you play objective or dominate in multiteam you could get this done in fewer than 4.
2500cR - One Spartan Army Kill 250 enemies in Firefight Matchmaking. Two or three matches of Gruntpocalypse in score attack will give you this.
2000cR - Hyper Lethal Vector Kill 100 enemies with headshots in Firefight Matchmaking. You should manage 100 headshots during the 2-3 matches needed to rack up 250 kills for One Spartan Army.
600cR - Blast Radius Kill 30 enemies in Firefight Matchmaking with grenades. Not sure if you'll get this or not during the One Spartan Army and Hyper Lethal Vector challenges. I tend to hang near an ammo dump and pick off grunts from a distance so I don't pick up many grenades. But you should be able to manage a decent number of grenade deaths in the initial play throughs and then play a standard score attack or whatever and go aggro to pick up extra grenades + die a lot to get the two you spawn with. Shouldn't take long.
And if you want to mix things up, you can obviously hop into some 4 player firefight matchmaking. I have fun with that in 3 or 4 match doses.
5850cr for 4 fairly easy challenges. Plus 2000cr for the weekly if you don't have it yet.