This is a multiplayer achievement and can only be done in ranked 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 fastest way to get 100,000 credits is to play Golden Gun mode. If you win, you get 1,000 credits a match so you have to win 100 matches. Even if you don’t win the round, you will still get a lot of credits. You do not need to have 100,000 credits all at once.
If you are having trouble getting credits, please use the Quantum of Solace Achievement Trading Thread