Easy Campaign CE:
After you've beaten the game, you can choose to restart the campaign. Trick is, every time you restart, you'll get paid more for completing the missions. You can get up to 9000CE for the last mission if you keep at this method. Useful if you can't find anyone to get online with!
Easy 1000+ CE:
Find a buddy and start a one round game with a two-player maximum. Start the round and kill yourself to end the match, and your partner will get the MVP award. Repeat, but this time have your friend blow himself to bits so you get the MVP. Getting an MVP gives you 170CE each time you get it, so if you repeat this, you'll have tons of CE. It's cheap and effective!
Extra Special Thanks to Revolution Reader Elite_seal_6!