This achievement requires you to earn a total of 10,000 cR. You will get credits for doing just about anything in the game and you will likely earn this by the time you beat the campaign.
If you don't want to do that you can go into matchmaking select Cooperative and then Score Attack. If you choose Gruntpocalypse and play it through fairly well you should get about 1,000 cR each time. It only takes about 12 minutes and it's super easy.
Also, you should check every day for the new challenges, they usually are pretty easy and will net you some extra cR pretty quickly on top of everything else you're earning!