NOTE: Only do this method if you wanna boost!!!!
Set up a Golden Gun match with eight people that are willing to trade off kills. Let each person take the gun and get 15-16 kills, while the rest run to the guy with the gun and let him kill you. Do this until everyone has 15-16. Then stay in the lobby and you can do it over and over until you get all 100,000 credits. Each match lasts 8 minutes and each person get 910-970 credits a match.
Note: If you buy all the weapons, grenades, gadgets, attachments, and golden weapons you will still not have 100,000. You have to play like 10-12 more games and the achievements will unlock. The fastest way is to boost and Golden Gun nets you the most points