This is a multiplayer achievement and can only be done in ranked matches.
You should be able to get this within your first 3 matches. At the end of each match, the credits you receive are 10 times what your score is. So if your score for a match was 50 then you would receive 500 credits. The easiest way to get 1,000 credits is to play a match of Golden Gun and win. To win a round of Golden Gun you need to get 100 for your score. Getting a score of 100 would mean you got 1,000 credits.
If you are having trouble getting credits, please use the Quantum of Solace Achievement Trading Thread.