This exhausts itself pretty quickly. The superior option is this: http://www.youtube.com/watch?v=rbeeA9COh2Ifor those looking for an easy way to rank up just go and play gruntpocalypse in score attack i get 1000+ credits which is not including commendation challenges. Another thing is the oni sword base airstrike pistol credit boost which can get you 30000+ credits in 30 minutes. If you do not know what that is look it up on youtube, there are plenty of vids on it.
All i can say is:
Credit Limit REACHed