I'm sure you're all aware of the Oni Sword base boosting spot that soon runs dry of credits, well there is another 'cheap' way of getting a lot of credits fast with little effort and legit!
1. Go to Matchmaking
2. Go to Score Attack
3. Play Gruntapocolypse and go for multikills and headshots using the DMR and pistol and you'll have Lt.Con in no time guys :-)
Let me know how you get on