If you're addicted to Bungie's credit system in Halo Reach, you've probably been paying close attention to the daily and weekly challenges they put out in the game. These challenges provide a ton of bonus credits for those that complete them and to this point, Bungie has done a pretty good job of balancing these between all the game modes including single player, versus, and co-op.
The new weekly challenge just went live - and while the first week was single player focused and the second week was versus, the third week's challenge is all co-op. For a sweet 2500 credits all you need to do is complete one mission of the game on Legendary with at least one co-op partner. Remember - if you're looking for someone to play with, especially since we're still waiting on campaign matchmaking - you can hit up the Co-Optimus forums!
Must...have...credits. Need...tactical...pad...for my Spartan!
Source: Bungie.net