First, you are going to want to go to http://halo.xbox.com...reer/challenges and login using your Windows Live ID. Then you should be at a page showing all of the current challenges, but if you scroll down you will see a "Create a Challenge" button; click it.
After you've done that, you are going to need to fill out these details.
For the skulls, you need to put them exactly as this:
Catch
Cloud
Famine
Thunderstorm
Tilt
Mythic
Cowbell
Grunt Birthday
IWHBYD
Then you need to make a second challenge that looks like this:
The skulls are the same:
Catch
Cloud
Famine
Thunderstorm
Tilt
Mythic
Cowbell
Grunt Birthday
IWHBYD
After you've done that, go into campaign using the necessary essentials. When you're in ONI: Sword Base, dash past the Elites and whatnot until you reach the Target Locator. Here is a video on what you need to do for it.
http://www.youtube.com/watch?v=6HUGgCkWetQ
Continue reverting to last save until you believe you have completed your challenges. The page on http://halo.xbox.com...reer/challenges will update after you end the game; you may need to refresh the page (at http://halo.xbox.com...reer/challenges) a couple of times and then turn in the challenge*.
Repeat this as many times as you want; you will NEVER ever get banned for doing this as it doesn't affect other players. This is how I get more than 100,000cR per hour.
*Your credits will be delivered after the three-hour time limit, or you can resume the campaign, end it, and get the credits that way; I don't know if it works that way (it works for me, though), but you'll definitely get the credits within three hours.