Where There's Smoke (500 Cr)- Kill 50 enemies in firefight matchmaking.
Fire When Ready (500 Cr)- Kill 50 enemies in multiplayer matchmaking.
It Was The Winter (3000 Cr)- Complete Winter Contingency on Legendary without dying.
There Are Many Like It (500 Cr)- Kill 50 enemies with precision weapons in the Campaign today.
Total- 4500 Credits
For the first two, just play. You'll get them pretty quickly regardless. If you only have time for one, then pick firefight, they're both worth the same. And now for It Was Winter. I'll be working on this one >_< For now all I can say is let the AI do the work, and that if your co-op buddy dies, you can still get the challenge. Then for the last one, play The Package and just use the DMR. When you get to the Scorpion, just start over. Happy credit-hunting!