Yea, I noticed that the amount of credits you receive dwindles shortly (I don't know why). At first I was getting around 200 credits for each hit but then it went down to around 34 credits. But then I just used the killing the elite challenge to my advantage and every time I got it, I would disconnect xbox live. Then reconnect it so the challenge would reset and I would repeat it. This way I was getting 1500 cR for each hit and I reached the level cap for today unfortunately, lol. =(