Aight. This little trick works for leveling, energy, and credits. Not only does it work for Contract Killer, it works for most games on the Market. If you have any experience with cheat engine, consider this nifty trick a version of that for android.
So here goes:
*YOUR ADFREE MUST BE DISABLED, AT LEAST MOMENTARILY*
First, open Market, install GameCIH 2.
Second, set your GameCIH hotkey (I used the search softkey)
Third, click the bottom advertisement to enable Hidden Mode. Wait 7 seconds on the page, and press your hotkey. If "Browser" is in red, you are in Hidden mode. If not, try again by closin out of GameCIH and repeating step 2.
Fourth, open up Contract Killer, and press your hotkey.
Fifth, tap input number, and input your current credits amount.
Sixth, press your back key to resume contract killer and earn at least 1 credit. (By "Liking Glu Mobile" or "Following Glu Mobile" or the method in the OP)
Seventh, after you have been credited, press hotkey again and type your new credit value. Repeat steps 6-7 until you have only 1 or 2 addresses.
Eighth, click those two addresses (one at a time) and change values to desired values (ex. 99999) and press MODIFY.
Final Step: Press back to resume contract killer and buy something that is worth credits. You should have a lot of credits.
good luck and have fun. If you need help just ask.